There might be hundreds of millions active users for the big apps, but I have been a part of an active and lively social network (general, not specialised) site with a few thousands users and one could already see powerful network effects emerge. Which is why I think mastodon does not have to dethrone anyone. The only thing that is needed is to acquire enough users that it will be self-sustaining. That would probably need a few million active worldwide users. It's not completely impossible, because right now there are enough people sensitive to one-platform problems, thanks to Zuckerberg and Musk.

If that happens, the big platforms have a big problem.