1. Ads cost more which raises the cost of doing business which raises the cost of goods.
2. It also makes certain borderline businesses infeasible -- there would be a whole class of businesses that don't/can't exist because they become unprofitable if you have to pay the ad monopoly/duopoly piper on top of normal expenses.
3. You could also argue that FB's continued dominance is another societal harm stemming directly from the alleged actions. If there wasn't an ad cartel, it would be easier to start new ad-based social media companies. (Wouldn't it be crazy if Google+ being neglected and then shutdown was a part of the negotiations between Google and FB?)
Lots of presumptions in the above, but I think a strong case could be made for each.