Come on, don't hand wave over the obvious. Think about how much it would actually cost to run a social media website that competes with the big social media on the core product of sharing and communicating with friends. It would be extremely realistic to build something that's both free and sustainable with just regular ads, as was done decades before.

(EDIT: to clarify, I don't mean to build an alternative monopoly, I mean to build alternatives that are big enough to survive as a business, and big enough to be useful; A few million users as opposed to the few billions Facebook and Youtube (allegedly) have)

The reason it's hard to imagine such a thing today is because the tech giants have illegally suppressed competition for so long. If Google or Meta were ordered to break up, and Facebook/Youtube forced to try and survive as standalone businesses, all the weaknesses in their products would manifest as actual market consequences, creating opportunity for competitors to win market share. Anybody with basic coding skills or money to invest would be tripping over themselves to build competing products which actually focus on the things people want or need, because consumers will be able to choose the ones they like.