Failure to grow, or at least to turn over users with time, is also a problem. Ultimately you end up with a dominant cohort which ages through the system.

For various reasons, Diaspora* seems quite prone to this. I have a number of suspicions as to why, though stagnent development and either intentional or incidental decisions which have made the platform almost perfectly opaque to general web search seem to be likely factors.

Mastodon, by contrast, seems to acquire a new major group of new adopters every year or two, that I've noticed.

(I'm active on both platforms.)