ActivityPub and the Fediverse of which Mastodon is part have absolutely resulted in viable alternatives. I use the network for probably 90% of my online social interaction. It is far superior to any alternative I've found so far, mainly because I can publish freely, cannot be banned from the network, nobody has to have an account to see what I say, and it is actually a network, not a website calling itself a network. I can think of no way to reach millions of people that is less restrictive than this network.