A site that scrapes, collates, and de-dups your friends' posts on every social media site, and then shows you the union of all of those posts as one feed.
This is the only way to break Facebook's back: to allow your friends' transition from one social network's data silo to another to be so gradual and effortless that you don't even notice it happening.
The thing that makes this difficult, of course, is not the coding, but the fact that if you succeed at it in any meaningful way, the sky will blacken with lawyers, and the data silos' spending on technical countermeasures will absolutely smother you.
It is hard, intentionally so, for people to quit a social network because that's where all their friends are and you can't get them all to move at once. But if it were possible for someone to move to a new service in such a way that neither they nor you lose that connection, then the barrier to switching would much lower. The services would have to compete on their merits rather than on your sunk cost.
But by facilitating this, not only would you be in violation of the terms of service of every site, you'd also be posing an existential threat to almost every aspect of their business model. They live for the lock-in. Touch that in a way that actually turns the Eye of Sauron upon you, and it won't go well.
Many of you are already bouncing up and down in your eagerness to go into the weeds with designs of how this could work at a technical level, but -- stop. It's a Small Matter of Programming, and that part doesn't matter at all. Unless you have a plan that solves "lawyers and countermeasures" problem, there's no point. You're looking for your keys where the light is good instead of where you dropped them.
So yeah, I said that step 2 is "and then a miracle occurs". This project is absolutely impossible. It will never happen, and social media cannot be fixed. Surprise!
But that step 2 miracle does have a name, and it is "antitrust legislation". It needs to be illegal for these companies to monopolize and lock in your data. It needs to be illegal for their TOS to prevent entry into the market of the kind of inventions that I'm talking about here. Interoperability and federation would need to be a legal mandate.
Anyway, good luck with that.