It looks like Mastodon will work out: lots of new users, still working well for me.
.@felixsanz W.r.t. backing up one’s Mastodon data: In the preferences, you can already export some of your data (but not your toots). I’d expect that there is more to come.
Also: it should be relatively easy to implement a tool that automatically syncs/backups one’s data; it’s just an Atom feed, after all.