Update:

ABSTRACT
(1) Despite the rise of bitcoin to around $14,000, monthly registered accounts in last 8 months are too far away from all time high at 238433, in Dec. 2017.
(2) Minimum and maximum of monthly registered accounts are 3 and 238433, in Nov. 2009, and December 2017, respectively.
(3) 50% of months (from Nov. 2009 to Aug. 2019) have total monthly registered accounts in the range from 2611 to 24472 (the interquatile range).
(4) 50% of those months have total monthly registered accounts above or below 10718 (the median).
(5) The lowest and highest medians (of months for each year) found at 148 and 66895, in 2010 and 2018.
(6) From peaks on 20/12/2017, BTC-closed price and monthly registered accounts decreased 91.3%, and 81%, respectively, on 15/12/2018.
(7) Since 15/12/2018 till 31/8/2019, BTC-closed price rose around 196%, while monthly registered accounts fell about 20%, respectively.

Converted dataset

Code:

. list id day month2 year date regacc btccloseprice, abb(30)

   +----------------------------------------------------------------+
   | id day month2 year date regacc btccloseprice |
   |----------------------------------------------------------------|
  1. | 1 2 11 2009 02nov2009 3 . |
  2. | 2 2 12 2009 02dec2009 8 . |
  3. | 3 1 1 2010 01jan2010 8 . |
  4. | 4 31 1 2010 31jan2010 22 . |
  5. | 5 2 3 2010 02mar2010 5 . |
   |----------------------------------------------------------------|
  6. | 6 1 4 2010 01apr2010 22 . |
  7. | 7 1 5 2010 01may2010 15 . |
  8. | 8 31 5 2010 31may2010 33 . |
  9. | 9 30 6 2010 30jun2010 325 . |
10. | 10 30 7 2010 30jul2010 159 . |
   |----------------------------------------------------------------|
11. | 11 29 8 2010 29aug2010 148 . |
12. | 12 28 9 2010 28sep2010 190 . |
13. | 13 28 10 2010 28oct2010 204 . |
14. | 14 27 11 2010 27nov2010 383 . |
15. | 15 27 12 2010 27dec2010 384 . |
   |----------------------------------------------------------------|
16. | 16 26 1 2011 26jan2011 938 . |
17. | 17 25 2 2011 25feb2011 2202 . |
18. | 18 27 3 2011 27mar2011 2737 . |
19. | 19 26 4 2011 26apr2011 4936 . |
20. | 20 26 5 2011 26may2011 14048 . |
   |----------------------------------------------------------------|
21. | 21 25 6 2011 25jun2011 6250 . |
22. | 22 25 7 2011 25jul2011 3903 . |
23. | 23 24 8 2011 24aug2011 2938 . |
24. | 24 23 9 2011 23sep2011 1983 . |
25. | 25 23 10 2011 23oct2011 1965 . |
   |----------------------------------------------------------------|
26. | 26 22 11 2011 22nov2011 1835 . |
27. | 27 22 12 2011 22dec2011 1812 . |
28. | 28 21 1 2012 21jan2012 2082 . |
29. | 29 20 2 2012 20feb2012 1901 . |
30. | 30 21 3 2012 21mar2012 2158 . |
   |----------------------------------------------------------------|
31. | 31 20 4 2012 20apr2012 1969 . |
32. | 32 20 5 2012 20may2012 2057 . |
33. | 33 19 6 2012 19jun2012 2040 . |
34. | 34 19 7 2012 19jul2012 2145 . |
35. | 35 18 8 2012 18aug2012 2419 . |
   |----------------------------------------------------------------|
36. | 36 17 9 2012 17sep2012 2661 . |
37. | 37 17 10 2012 17oct2012 2958 . |
38. | 38 16 11 2012 16nov2012 2763 . |
39. | 39 16 12 2012 16dec2012 2611 . |
40. | 40 15 1 2013 15jan2013 3320 . |
   |----------------------------------------------------------------|
41. | 41 14 2 2013 14feb2013 4933 . |
42. | 42 16 3 2013 16mar2013 13816 . |
43. | 43 15 4 2013 15apr2013 18162 . |
44. | 44 15 5 2013 15may2013 11318 115 |
45. | 45 14 6 2013 14jun2013 8095 103 |
   |----------------------------------------------------------------|
46. | 46 14 7 2013 14jul2013 6703 96 |
47. | 47 13 8 2013 13aug2013 5505 107 |
48. | 48 12 9 2013 12sep2013 4137 135 |
49. | 49 12 10 2013 12oct2013 5632 134 |
50. | 50 11 11 2013 11nov2013 26804 333 |
   |----------------------------------------------------------------|
51. | 51 11 12 2013 11dec2013 28776 958 |
52. | 52 10 1 2014 10jan2014 29576 833 |
53. | 53 9 2 2014 09feb2014 29839 660 |
54. | 54 11 3 2014 11mar2014 22534 623 |
55. | 55 10 4 2014 10apr2014 18065 416 |
   |----------------------------------------------------------------|
56. | 56 10 5 2014 10may2014 13071 450 |
57. | 57 9 6 2014 09jun2014 12180 649 |
58. | 58 9 7 2014 09jul2014 10561 626 |
59. | 59 8 8 2014 08aug2014 10925 590 |
60. | 60 7 9 2014 07sep2014 9418 485 |
   |----------------------------------------------------------------|
61. | 61 7 10 2014 07oct2014 8218 332 |
62. | 62 6 11 2014 06nov2014 7974 343 |
63. | 63 6 12 2014 06dec2014 8136 376 |
64. | 64 5 1 2015 05jan2015 23256 277 |
65. | 65 4 2 2015 04feb2015 36913 228 |
   |----------------------------------------------------------------|
66. | 66 6 3 2015 06mar2015 33500 269 |
67. | 67 5 4 2015 05apr2015 8450 253 |
68. | 68 5 5 2015 05may2015 8577 235 |
69. | 69 4 6 2015 04jun2015 10303 226 |
70. | 70 4 7 2015 04jul2015 11773 256 |
   |----------------------------------------------------------------|
71. | 71 3 8 2015 03aug2015 9725 282 |
72. | 72 2 9 2015 02sep2015 7644 228 |
73. | 73 2 10 2015 02oct2015 68601 238 |
74. | 74 1 11 2015 01nov2015 44059 316 |
75. | 75 1 12 2015 01dec2015 37106 379 |
   |----------------------------------------------------------------|
76. | 76 30 1 2016 30jan2016 42233 378 |
77. | 77 29 2 2016 29feb2016 24213 438 |
78. | 78 30 3 2016 30mar2016 19336 413 |
79. | 79 29 4 2016 29apr2016 16031 449 |
80. | 80 29 5 2016 29may2016 19935 526 |
   |----------------------------------------------------------------|
81. | 81 28 6 2016 28jun2016 12582 651 |
82. | 82 28 7 2016 28jul2016 10646 656 |
83. | 83 27 8 2016 27aug2016 9986 576 |
84. | 84 26 9 2016 26sep2016 9844 607 |
85. | 85 26 10 2016 26oct2016 10718 670 |
   |----------------------------------------------------------------|
86. | 86 25 11 2016 25nov2016 10691 738 |
87. | 87 25 12 2016 25dec2016 12429 899 |
88. | 88 24 1 2017 24jan2017 13664 909 |
89. | 89 23 2 2017 23feb2017 13893 1140 |
90. | 90 25 3 2017 25mar2017 15276 924 |
   |----------------------------------------------------------------|
91. | 91 24 4 2017 24apr2017 20585 1223 |
92. | 92 24 5 2017 24may2017 31381 2457 |
93. | 93 23 6 2017 23jun2017 35866 2745 |
94. | 94 23 7 2017 23jul2017 40726 2792 |
95. | 95 22 8 2017 22aug2017 45026 4002 |
   |----------------------------------------------------------------|
96. | 96 21 9 2017 21sep2017 70477 3858 |
97. | 97 21 10 2017 21oct2017 95568 6080 |
98. | 98 20 11 2017 20nov2017 144762 8014 |
99. | 99 20 12 2017 20dec2017 238433 17132 |
100. | 100 19 1 2018 19jan2018 136872 11818 |
   |----------------------------------------------------------------|
101. | 101 18 2 2018 18feb2018 107237 10798 |
102. | 102 20 3 2018 20mar2018 87087 8564 |
103. | 103 19 4 2018 19apr2018 89659 8149 |
104. | 104 19 5 2018 19may2018 83999 8270 |
105. | 105 18 6 2018 18jun2018 72430 6569 |
   |----------------------------------------------------------------|
106. | 106 18 7 2018 18jul2018 61360 7338 |
107. | 107 17 8 2018 17aug2018 41451 6441 |
108. | 108 16 9 2018 16sep2018 41516 6464 |
109. | 109 16 10 2018 16oct2018 35335 6551 |
110. | 110 15 11 2018 15nov2018 31502 5675 |
   |----------------------------------------------------------------|
111. | 111 15 12 2018 15dec2018 20733 3253 |
112. | 112 14 1 2019 14jan2019 20046 3596 |
113. | 113 28 2 2019 28feb2019 16202 3854 |
114. | 114 31 3 2019 31mar2019 18932 4105 |
115. | 115 30 4 2019 30apr2019 17910 5350 |
   |----------------------------------------------------------------|
116. | 116 30 5 2019 30may2019 24472 8319 |
117. | 117 30 6 2019 30jun2019 22357 10817 |
118. | 118 31 7 2019 31jul2019 18913 10085 |
119. | 119 31 8 2019 31aug2019 16452 9630 |
   +----------------------------------------------------------------+

Plot:

Statistics:
Mean +/- standard deviation: 22031 +/- 33740.
Median (Interquartile range): 10718 (2611 - 24472)
Min - max: 3 - 238433

It means 50% of months (from Nov. 2009 to Jul. 2019) have total monthly registered accounts in the range from 2611 to 24472 (the interquatile range).
Additionally, 50% of those months have total monthly registered accounts above or below 10718 (the median).

Code:

. tabstat regacc, s(n mean sd p50 p25 p75 min max) c(s)

variable | N mean sd p50 p25 p75 min max
-------------+--------------------------------------------------------------------------------
regacc | 119 22030.18 33740.01 10718 2611 24472 3 238433
----------------------------------------------------------------------------------------------

Statistics for year-specific months:
Let's look at median (p50) of monthly registered accounts for each year.
The lowest and highest of medians found at 148 and 66895, in 2010 and 2018. (for possible explanation, please read OP).

Code:

. tabstat regacc year10 year11 year12 year13 year14 year15 year16 year17 year18 year19, ///
> s(n mean sd p50 p25 p75 min max) c(s)

   variable | N mean sd p50 p25 p75 min max
-------------+--------------------------------------------------------------------------------
   regacc | 119 22030.18 33740.01 10718 2611 24472 3 238433
   year10 | 13 146 144.3185 148 22 204 5 384
   year11 | 12 3795.583 3560.192 2469.5 1900 4419.5 938 14048
   year12 | 12 2313.667 353.35 2151.5 2048.5 2636 1901 2958
   year13 | 12 11433.42 8798.91 7399 5219 15989 3320 28776
   year14 | 12 15041.42 8092.689 11552.5 8818 20299.5 7974 29839
   year15 | 12 24992.25 19299.52 17514.5 9151 37009.5 7644 68601
   year16 | 12 16553.67 9350.784 12505.5 10668.5 19635.5 9844 42233
   year17 | 12 63804.75 67452.29 38296 17930.5 83022.5 13664 238433
   year18 | 12 67431.75 34995.66 66895 38393 88373 20733 136872
   year19 | 8 19410.5 2840.515 18922.5 17181 21201.5 16202 24472
----------------------------------------------------------------------------------------------

Note that figures for year of 2019 (year19) has just counted for the first 8 months.

Discoveries
I will compared percentage of changes of registered accounts and BTC close price at the same days that we have in the dataset above.

Peaks:
Registered accounts and BTC-closed price peaked at 238433, and 17132, respectively, on 20/12/2017.
From that peak, BTC dropped to lowest (just lowest in the dataset), at 3253, on 15/12/2018. That drop equals to 91.3-percent fall.
In the same period, monthly registered accounts dropped from 238433 to 20733, that equals to 81-percent fall.

Code:

. di (238433-20733)/238433*100
91.304475

. di (17132-3253)/17132*100
81.012141

Movements from bottoms
During the same period (from 15/12/2018 to 31/8/2019), from bottoms (acorrding to the dataset), registered accounts and BTC close price moved in opposite directions.
- Monthly registered accounts fell from 20733 to 16452, that equals to 20.6-percent of decrease.
- In contrast, BTC-closed price rose from 3253 to 9630, that equals to 196-percent of rise.

Code:

. di (20733-16452)/20733*100
20.648242

. di (9630-3253)/3253*100
196.03443

It means the forum has lots of changes in the way its operate, and affected on monthly registered accounts. From bottoms, monthly registered accounts have fallen dramatically (~20%), compared to the recovery of bitcoin (~196%).
Within the last 8 months, from 14jan2019 to 30june2019, there are six months, in which monthly registered accounts dropped to below 20733. They are February, March, April, July, and August, at 16202, 18932, 17910, 18913, and 16452, respectively.