I think social media is a solved problem at this point, you'll need something radical or game changing to actually break through in this market. Combined with the fact that the fediverse is inherently much more difficult to monetize I don't see many companies taking on that challenge.
FOSS projects might though, but they tend to grow too slow to be disruptive.
If you're thinking of it in market terms, then it being a "solved problem" should mean that it's effectively a commodity and nothing radical or game-changing is needed at all to eventually break the monopolies and win all the market share. All that's needed is to offer the same old thing at a slightly lower price, and wait for people to catch on.
But I disagree; there are plenty of unsolved problems.
I'm thinking the same way smartphones are solved where only small increments of improvement happen. Radical changes happenen, like folding phones or the rise of Tiktok. Some have long lasting problems like the former, but the latter managed to pick a fight with the giants and come out on top.
Back to market terms, they're mature but new players have proven to disrupt the market. When the general public start caring about privacy, federated social media will rise. Seeing how that is quite a politicised thing, progress will be slow. I'd love to be proven wrong though.