Mastodon is cool technology. The problem is Facebook fills a hole that Mastodon does not. Facebook connects people in real life. Folks from varying distances are able to keep up with their friends and family. There's a built-in desire to monitor their conversations. They're rewarded with "reactions". Facebook provides value for a very large group of people.

A relatively small group of people saw a problem with Facebook. Of course they have to have some underlying goal to make money. So they set out to create a federated system anyone can host that doesn't depend on some central company that's just out to make a buck. It's a noble goal, but it solves a problem that most users of Facebook simply don't care about. It doesn't do the thing that Facebook does, so it's a non-starter.

I'd also add that Mastodon is more of a Twitter replacement than a Facebook replacement. Facebook is better at isolating your feed into your group of "friends". Mastodon is more of a river of information from folks you follow (many of whom you will never meet in real life.)

If you want to replace Facebook, first figure out what people value about Facebook and expand on that. You'll need to be better than they are (at least until people determine they're that awful.) Then look at how you can federate it or achieving whatever other goal you have.