Except that, because of network effet, people will keep using the service no matter how bad it becomes, as long as it keep a few basic functionalities?

This is the problem. You are projecting what you consider bad to the majority of users. What you call "a few basic functionalities" is all that people want; they don't care about the rest. They are not staying there because of "network effect", they are staying because their "few basic functionalities" are satisfied.

They will demand change only when these "few basic functionalities" are violated (for example, ads that make it impossible to watch videos). This is inevitable, but big tech will try to delay it with subscriptions etc.

The majority of people don’t see a problem in capitalism either, does it mean one should stop advocating against it.

Facebook is a company that harvests users’ data and attention, under the hood of some social networking capabilities. Having a facebook account is not a sin, but it is exposing oneself to that, as well as pressuring one’s friends into doing the same, as I mentioned above

Unlike your argument against Facebook, the advocacy against capitalism is not a moral one; it is a recognition of the irreconcilable contradictions that arise from private ownership and socialised labour. The majority of people will eventually find themselves in a position where it is necessary to fight capitalism. In the same way, the majority of social media users will find themselves in a position to advocate for better alternatives once the contradictions deepen. And when they look for alternatives, federated social media will always be inferior to centralised solutions.

People leave reddit for lemmy because their "few basic functionalities" were violated. Moving to a federated solution can somewhat solve it, but only at the cost of problems inferior federated software brings. Nobody is willing to tolerate them, except tinkerers who want to play with federated software.

It's not the evilness of capitalists that will bring the revolution, but the internal contradictions themselves.

I don't outright reject the idea that network effects exist; they do. However, if your argument about network effects is correct, it means that big tech will never be replaced; no amount of alternative federated software you write will be able to replace the "network." This would imply that big tech has found this small trick that solves all other contradictions and they have reached an ideal stage with no contradictions like communism, in reality the network effect is just a side effect that you observe because of the differences between your needs and the needs of the majority.

Federation aims at that, by allowing to build a big network without a single person being able to impose marketing choices over the whole network

Nice dream. But in the real world, the majority of fediverse users are centralised on a few servers, and a minority of admins decide the shape of the network. You have become the same thing you wanted to destroy.