I mean given the scope of services that went down and the huge disruption globally that resulted from it, not just domestically...yeah.
Are they a hard-monopoly? No. Have they created, like many companies, soft-monopolies where they're the primary/only major player in a space? Absolutely. Are they part of what can easily be viewed as an oligopoly? Absolutely. Do they have a history of anti-competitive practices like constantly buying out other social media platforms like Instagram to prevent them from becoming actual competitors? Absolutely. Have the recently leaked documents and testimony painted a picture of a company that's become far, far too large to even manage itself and happily and knowingly lets its platform do massive amounts of direct harm globally, including by acting as a platform to promote hate and organize ethnic cleansings without much of a check from Facebook (whose reporting admits that they only action 2-3% of rule breaking content)? You bet your ass.
Porque no los dos?