Why? The available computing power and bandwidth are orders of magnitude more plentiful / cheaper than in the 2000s, too. I can't think of any technical reason why we couldn't have social in today's world media without advertising money.
The main non-technical reason why you can't run Facebook on the cheap is that it's expensive to respond to regulatory and PR pressures they're under. You need an army of lawyers, lobbyists, and compliance people in almost every country on the planet.
But that's in some respects a product of consolidation that we never really needed in the first place: we don't need every human on the planet on the same social network. Social circles are small and the only reason to have everyone under the same roof is if you want to be the gatekeeper for the world's ad targeting data.
The scrutiny is also a product of amount of money involved. No one is exerting as much pressure on Signal, Mastodon, etc, precisely because they're not trillion-dollar companies.