> If we're lucky, that means a federated, open, mostly-ad-and-suggestion-free open source social media ...
It's never going to happen.
Federation is a solution to a problem no one actually cares about. Obviously "no one" here is somewhat hyperbolic but so few people actaully care about this you may as well round it to zero.
We haven't had an at-scale federated network since POTS (and, by extension, SMS). That's a historical artifact. And look at all the problems POTS/SMS has with spam, caller impersonation, robocalls, etc. Now part of that is due to these being open addressable networks rather than the opt-in networking that has become dominant now.
But to say that's th eonly reason would be to miss the point that any federated systems will always have bad actors.
In the 2000s we had XMPP as an effort to make IM services interoperable. Remember how Google got flak for removing the ability to talk to non-Google services? That was mostly misplaced because the reason it was removed was that Microsoft allowed its IM users to talk to Google Chat but not the reverse.
Spam exists on POTS/SMS largely because smaller exchanges "launder" bad traffic with good traffic (and charge higher rates for it) to make it harder to filter or even identify who the bad actors are.
As for open? There's really no incentive for this except from the "losers" (in the market).