My own take on it is that growth is not very important in terms of how a network develops. The only truly successful growth is that which happens completely organically. Worrying about why one service has 'stopped' growing is pointless. Those who are unhappy jump ship - those who remain are likely people who are never going to and/or bots or influencers who aren't interested in being part of a community just finding a way to exploit it.

I would propose so-called 'smaller' networks (such as the fediverse) concentrate on quality not quantity. That has the duel benefit of making the experience for current users even better and makes the network attractive to those who are outside looking in.