Sorry but I think mastodon is DOA. The user experience for on-boarding is atrocious. Think of what the average user would do if you tell them to join mastodon.

First if you search for it and go to the first page it talks about open source (no average user knows what that is; some have vague ideas but in my experience none of them know what it is beyond the very rare person who heard a report on TV and thinks it's "nerds doing something for free"). Then it talks about "blah blah blah social should be decentralized blah blah blah you cannot sign up here because we believe in decentralization!" (paraphrased, of course)

Now the user, the average user who very likely doesn't understand why this pure decentralized thing is better than Facebook or Twitter since he or she can go to one one place, sign up and immediately start posting, has to somehow find somewhere else to sign up because of some purity stance?

Look I'm all for decentralizing things. But it's never, ever going to see the growth of average users having this much friction. When you want to take growth away from highly popular incumbents you need a way to have less friction than them (at least usually, in my experience). Decentralizing, at least in this form, adds an order of magnitude more friction.