It's not actually illegal to be a monopoly but to illegally abuse your market position leading to anti-competitive behavior.
Unfortunately, it's never enforce properly as the FTC doesn't have any teeth.
I think the only way we're going to solve this is to actually TAX these companies appropriately. This is one of the major reasons they metastasize.
I’m still confused as to which “market” is harmed: advertisers? Facebook users? other social networks?
If the market is Facebook users, what’s the precedent in asserting this level of harm to a market that consists substantially of non-paying users of an inessential service?
Several people will say, "try not having a Facebook account and participating in X," where X is some mandatory part of your life, such as your children's schooling, keeping up with relatives, job-related stuff, social clubs or organizations, etc. It might not be impossible, but it's an undue burden in some cases.
It's impossible to keep up with college affairs without Facebook, as a Facebook group is the primary method of communication and organization.
It was even worse before the pandemic, since there were no MS Teams or Google Classrooms for each subject. If an assignment was released, it was the representative's Job to male sure it reaches everyone. Guess what did they use?
Oh the assignment is delayed? TA announces on Facebook group. Midterms Time table? College Facebook page. All college-wide announcements were made through its Facebook page.
Facebook is practically essential for college life. You could say it's the College's fault, but that's not our discussion. What's important is that I can't delete my Facebook without causing a lot of pain in the ass for myself.
Even with scare quotes it's not a multiple: the social network of Facebook users is an advertising market. You don't have to be a snot-nosed growth hacker to see that FB has a monopoly position in advertising.
Here's a test: does FB have any reason to care about the quality of whatever is being advertised in any one ad? Where are you going to advertise if FB keeps putting your ads for your doctor's office next to ones for a child-rape rock opera and/or "COVID-19 is a hoax" seminars? What is your recourse, switching your advertising to Parler and LinkedIn? NYTimes dot com? The 72 next most popular social networking sites around the world? Surely VKontakte runs a clean house.
Again, look at railroads. In the Trust era, trains were not the product, were not the market. Transportation was.
Here's a nice little rundown: https://twitter.com/jason_kint/status/1336813617736507397
We had a few decades of true antitrust, but the efforts to undermine it were successful. Check out Goliath by Matt Stoller for a decent history.