Most Mastodon users are on Twitter anyway, many of them mirror their accounts. Being on Mastodon does not do much except a slightly more attractive GUI. Unless you use TweetDeck... which Mastodon's default UI (badly) imitates.
Mastodon has just another wave of "Twitter sucks" users. This is one of the problems of GNU Social/Quitter, ello, Diaspora and Mastodon: They try to attract disappointed Twitter (and/or Facebook, at least Diaspora does that) users instead of a new group of users. The only "Twitter alternative" that does not just try to get Twitter users to tweet on a different platform instead would probably be twtxt - but who (except me and a few others) uses that?
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Mastodon has its problems though: