I guess its because of the merit system, after this system introduce many cheaters and spammer's lose their hope to rank up and there are lots of users get negative trust because of merit abusing most of the users with negative trust quit the forum and I think some of them now are focusing on 1 account.

Im not shocked that the active users count now is lower than before because before the merit system introduce to the community there are users with 20 account's and higher that only owned by 1 you can see it on the topic created here in meta https://bitcointalk.org/index.php?topic=2896910.0. Im sure after getting negative trust on their accounts they are not interested anymore to that account and abandoned it.


This is the benefit of the merit system in the forum we can now see the real count of legit users and the quality of post of other members have improved.
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Another view in this that affects the decreasing count of real active  forum member that I think is because of the bad market situation, many investors joined here last year with high expectations on bitcoin and some of them are now quitting the crypto world because they are seeing the market falling down especially those investors who just invest without knowledge about cryptocurrency.