Whenever Twitter is #down, people will recommend Mastodon as a better alternative since it's a federated system which can't go offline as a whole.
While #federation is great in terms of independence and resistance to censorship, I don't think #TwitterDown is the best argument to attract new #Mastodon users, since – for the individual user – it wouldn't make any difference whether #Twitter or the respective Mastodon home server is #offline.