Actually, anything COULD replace Facebook, but let's look at what the requirements would be from a user's perspective;
Ubiquity - Has to be everywhere, or maybe not on day one, but there has to be a clear, believable path to it.
Free to use - Sure, Facebook is Ad supported, a true peer-to-peer system wouldn't be. No on is supporting 'bittorrent' per-se, it doesn't cost anything to push content, download, other than having the required 'bandwidth', and the ISPs carry that, like it or not.
Interoperability - the system would have to be able to work with existing social media and communications platforms, such as text, email, voice, video, forum chats, and of course, the competition.
It has to be trusted - People will use it and trust it, if everything that is posted is traceable back to the originator. That would be a key differentiation. This way, there would be no clandestine government meddling or fake news, it could always be traced down, and flagged by requirement number 5.
It must support reputation scores, and REAL measures of influence, not ones that can be bought to push to the top of the list. And lastly a hard one-
It must enforce responsibility and consequences. If someone gets down-voted enough or their reputation falls to say, criminal levels, they can get muted automatically by anyone who chooses to ignore people with poor reputation scores, and there has to be a long, long road o redemption. (This would imply only individuals could censor, not governments).
If all 6 of those can be satisfied, and provide a PUBLIC face for real sharing person to person, without BOTS, the new social media platform COULD replace Facebook, and the rest of GAFA would also be subject to replacement as well.