>Much like almost everything else from that time, they simply just outgrew and replaced their core demographic with a group that are much easier to exploit.
I assume they found much more insular communities either in the form of obscure subreddits or other communities (like here for example). It might not be the case of them actually leaving, it is just that for every user like that, there are 10000 new users who came from some place like facebook.
Where did the core demographic go?