Current anti-trust court rulings are based on a change of thought from the 1970s. While I'm not that old, it's rather more modern than I would have expected. Prior to the 1970s anti-trust was about competition even if that competition was worse. After the 1970s the courts changed their thinking and decided it was all about moving towards cheaper offerings. If something made someone cheaper, that was better for society and thus they were allowed to do anything that made them cheaper.
My thought is we keep that same concept, but we realize that services like Facebook specifically aren't zero cost. They're negative cost. Facebook makes money on each of their users. Let's pretend for a moment that a company makes $10 per month from every user. That service isn't $0, it's negative $10/mo. If another company can offer the same service for less than negative $10/mo (ie, make less money off their users) then they should win. Think of the values in absolute value terms, rather than just a normal 0+ number line. If/When Facebook or another company where to propose a merger, then the merger would be thought of if it means the company makes less money per user or more money. If the company makes more money per user, then the merger would be denied. That's it. Same exact system we have now, with no changes to the law, but the courts and government polices would look at the cost being closer to zero, rather than it being "free".
Sadly, whenever I try to explain this to someone, they don't seem to understand my idea here.