There is an exchange for value between Google (FB, etc.) and consumers; the fact that it's an in-kind exchange rather than money doesn't prevent anti-trust analysis from being applied.
There's also the possibility of monopoly in some advertising segment, sure.
I suppose then the anti trust argument would be that they are able to 'ask' for too much private data. It seems to me a first step would be gaining price transparency. Who is the cheapest (asks for the less data).
Actually if your theory holds water, isn't there scope to argue that the consideration (your data) isn't properly defined in the contract, and keeps changing (via defaults, and technological progress). Could then this not then be dealt with via contract law and consumer protections?
I suppose you could make the argument that fb/google are monopolies because the advertisers have to go through them to reach us. It isn't clear to me though that this is the problem we actually want to solve. This leads me to believe that anti monopoly laws might not be the correct tools for dealing with these companies.