Do you accept that facebook is harmful to the world

Yes. But I believe it is harmful because of the way it has been leveraged to crowd out the public sector and strangle competitive private alternatives. It has become one wing of a massive tech sector cartel.

If you accept that, it’s a small step to “They benefit from having more users on their platform”.

That doesn't logically follow. No more than saying "Building more highways is bad for the environment, ergo the highway administrators benefit from having more cars on the road." You're looking at a problem of induced demand and concluding the problem is on the demand-side of the equation.

Now, getting one family to stop using facebook is a drop in the bucket.

It's one node in a massive web. And it's easy to say "Well, you have to do your part because ". But mostly it's just some random asshole on the internet telling me not to use my telephone because AT&T is run by a richer set of random assholes. There's no material benefit to me and no collective coordinated action that I'm seriously participating in.