It is even harder to argue this when Facebook's customers are advertisers, not users (which are a resource). Facebook does not have a monopoly on advertising. But this is the best avenue to show it does consumer harm, which is the legal critera for a monopoly. Here, "consumer" would be advertiser. How Facebook treats advertisers is going to determine the legality of any monopoly claim, regardless of whether people intuitively believe Facebook is "too big". To make that be a criteria for breakup, one would need to change the law, not issue any directives from administrators.