I'm not the most on point as far as keeping up with the internet so possibly it is actually happening, but I have not yet identified a direct challenge to Facebook from the fediverse that has been settled on by those already here.
I was on Mastodon for a while but realized I hate Twitter-style interactions, as much as I enjoyed posting about all the stuff I'm into - as the Twitter people kept coming like waves of Saxons with funny hair on Britain's fair shore, I got into some supremely silly arguments and then got out. I didn't bother to wait for them to burn my village, they're welcome to it.
I'm now giving Lemmy a go, because as far as participation in platforms, I lasted longest at Reddit, though I was gone long before the recent exodus. Hopefully my dogs, cats, plants and microcontroller projects will get some love from The Internet's Good Strangers here.
But I was, in the early days, quite an avid FB user and considered it unleaveable until 2016, at which point I realized it was not just leavable but likely to get us all killed. I still have a (good parts of) Facebook-shaped hole in my online life, which is where all my real friends and relatives used to hang out for my daily perusal, and where I could send out my various snarks and know I was amusing at least one or two people who genuinely found my antics delightful. I'm not a troll but I'm definitely a Grouch, and even Oscar needed a hug every now and then.
So given that most of us are here because we recognized the cycle of enshittification at some point and decided to make a different choice, and given that we've so quickly embraced replacements for every other big silo, and given again that most of us were probably once on FB and used it to be connected to our real people... why have we collectively shied away from even offering a viable Facebook alternative?
Whenever I ask my more "woke" friends why they're still there, it nearly always seems to be that their old relatives are all there. I can see that that would be a great challenge, to move them off of that pablum-crack. Maybe the Secret Council Of Woke Fediverse Elders is using all these lesser platforms as gamergate-like test runs to iron out the kinks in federation. Perhaps even the seeming willingness of Mastodon admins to let Meta poke their tentacles in the door is entirely a feint - perhaps Mastodon was never intended to be kept in the first place, but rather, is just a honey pot to gather important battlefield notes for the coming attack!
maybe?
You've basically elucidated my working hypothesis as to why there is not a prominent FB competitor already - someone already mentioned Friendica, and I had heard of that, and I think gnusocial is another one. But this absolutely is precisely why this will be the hardest to replace. Every other platform just needs users, this one needs to connect to the people you actually care about, and indeed, our grandparents and such are not gonna be enthusiastic about something that's more difficult to use.