I think the OP should have used several graphs that could be used to compare the trends over a bigger period of time as it would allow a greater understanding of how the number of posts per day were affected. Clearly 300,000 active users versus 57,000 does make a difference and will contribute to the number of posts being made.
How did you define active, was it any member that logged in the previous 3/6/9 months?
When I launched
https://bpip.org, there were over 300,000 active members. Now there are 57,000. That prob has a lot to do with it.