"Twitter" and "tweet" sound very light and airy, almost musical; the theme of birds chirping lines up well with the format of seeing many short messages. "Twitter" is also largely an abstract word without much prior association.

A mastodon is a massive, hairy, smelly beast. It's also an extremely well known heavy metal band. For a Twitter clone it makes absolutely no sense. "Toot" as well has some existing connotations I wouldn't think they would want to associate with...just seems like a total non-starter.

Facebook came from actual face books at universities. It a brilliant name because the entire point was to see people you might want to sleep with. It wasn’t about sharing photos with mom.