They use what their friends use. And won’t coordinate a migration for nothing. The alternatives aren’t a distinctly better thing, but instead look like another brand without the people. Most of the innovation has been in “non-functional requirements” (like not screwing your users) and “like X, but on the Y”, which we know enough to laugh at for patent novelty, but not for social network.

People do migrate for new and different things, like Instagram. And then those get hoovered up.