I'm not sure Musk knows what he's doing, this feels like a personal vendetta. I'm also not sure he has any likelihood of succeeding in the first place, I wouldn't necessarily assume anyone is going to take him up at that price. And (quite frankly) I don't think that Musk is a very good free speech activist. I think he's regularly hypocritical about free speech and regularly engages in his own forms of censorship; I d…
I didn't even know this existed until you said it, which doesn't bode well for mass adoption given that I'm someone who's on the internet daily.
Moreover, Mastodon.com is owned by a forestry machine business, and googling Mastodon gives a first result for a rock/metal band. I think "Mastodon.social" might be about as bad as a web address can get for mass appeal / catchiness.