Transparency is part of the problem. It makes the anti-competitive practice easier to carry out because customers don't know they're getting screwed. But there's still a potential anti-trust issue even with perfect information.
Suppose Facebook doesn't want anyone using their competitors, so they subsidize the cost on some ISPs that then block their competitors. The customers of those ISPs are 15% of the market, and they know the other competitors are blocked, but they want the discount. Then the other 85% of people have to use Facebook in order to communicate with anyone on one of those ISPs, and social networks have a network effect, so now everybody is stuck on Facebook even if they don't use one of those ISPs, because they know somebody who does. This is anti-competitive and so an anti-trust problem.
I think it's much more likely people stop using Facebook in that condition. People may be "stuck" with Meta because everyone is on it but the situation you're describing is a big difference between zero friction to make an account and join everybody else and change your ISP so you can talk to your grandma and look at cats on instagram.
I'd rather have choice and transparency and see if the situation you've described arises. It sounds completely unrealistic to me and we don't have to make laws and regulations cover every single edge case right away, they can be modified as we go.
You've failed to understand the example. Nobody would have to change ISPs to use Facebook because Facebook paid off the ISPs for some form of exclusivity. It is Facebook's competitors who would struggle wity user acquisition because not only do you have to convince users to change ISPs to use your competing platform, but you have to convince all your frienda and family to switch too if you wanna be able to call them.
> It sounds completely unrealistic
You are incredibly naive then. This sort of thing regularly happens all around the you. Kickbacks, exclusivity and companies colluding for competitive advantage is commonplace, not unrealistic.
Transparancy (at least how I see it) is matter of culture.
For example, if you open up ISP accounting books to the public, creative people will find ways to hide things. Instead of line item “10M € kickback from facebook”, there will be “10M € sale to facebook”.
Suppose Facebook doesn't want anyone using their competitors, so they subsidize the cost on some ISPs that then block their competitors. The customers of those ISPs are 15% of the market, and they know the other competitors are blocked, but they want the discount. Then the other 85% of people have to use Facebook in order to communicate with anyone on one of those ISPs, and social networks have a network effect, so now everybody is stuck on Facebook even if they don't use one of those ISPs, because they know somebody who does. This is anti-competitive and so an anti-trust problem.