Mastodon can be successful for the people who use it even if it doesn't "make it big" enough to dethrone Twitter, Facebook, etc.
There are longstanding challenges to marketing something like Mastodon. Call it the "fediverse" or "PubSubHubBub" and I think people's eyes glaze over. But it doesn't have to appeal to everybody, and it might be better if it doesn't... Wouldn't it be great if we could undo the "Eternal September?"