Asserting "X is not really a monopoly" is not particularly relevant because that's not the standard which matters. It's perfectly plausible for a not-really-a-monopoly with a large but shrinking market share to abuse a dominant position in a particular market (often defined more narrowly than you might think) in ways that violate anti-trust laws.
In particular for US law, this part:
> having a dominant market position and using that for anti-competitive behavior
For the EU, the question can stop there; in the US, there's an additional burden of proof that anti-competitive behavior harms the consumer.
Google is allowed to build a giant search empire because so far, courts have agreed that while it's hard to compete with Google, the consumer is not harmed because a competitor is always a click away and barrier to switching is low.
After a certain point, maintaining size means sustaining an onboarding pipeline. Tobacco and alcohol ran into this issue, espeically as they were busy killing off their longest a-term and most loyal customers. Facebook's not quite there.
Yet.
That's entirely irrelevant to the "is Facebook a monopoly" question. If Facebook is losing market share to a very different social network, this still shows Facebook doesn't have absolute control of it's customers and market. I would strong suspect that every rising social network is going to be significantly different from the social network it captures users from.
Facebook competes with linked-in for business stuff, Amazon and craigslist for selling, Tik Tok for youth attention etc.
The main thing is an argument for Facebook being powerful, influential and even abusive isn't by itself an argument for it being a monopoly as such.