This is a constant misconception in these discussions but antitrust litigation and penalties don't require there to be a monopoly.

The allegation here is anti-competitive behavior. There are plenty of things you an do that are anti-competitive and illegal without actually achieving a monopoly. The classic example is price fixing but there are many others.

Unsurprisingly, the actual linked article in this thread spells out these allegations of anti-competitive actions on Facebook's part.