FB isn’t a monopoly. It’s a business of scale. They don’t control any limited resource that would prevent others from competing with them.
Myspace would beg to differ.
Someone could start a website and build data centers but google and others would block searches, comments about new competitor, agreements with other sites not to use their search. The article below was longer earlier in the day but is now a snippet of what I saw.
Google had agreements blocking competitors
https://www.fastcompany.com/90322678/google-hit-with-another-eu-antitrust-fine-the-grand-total-now-comes-to-e8-2b
Google didnt allow its AdSense customers to feature rival search engines on their sites.
Another article about fine
https://www.nytimes.com/2019/03/20/business/google-fine-european-union.html