You're asserting that X is happening, so the burden of proof is on you.
However, I think the lack of any forementioned lawsuits is strong evidence that the situation is at least a little more nuanced.
If an internet commentor came up with the idea for a lawsuit with all of 5 minutes of thought, then I promise you that the teams of lawyers that the advertisers employ have also already thought of that, and Facebook's lawyers have also already thought of that and have made sure to limit illegal behavior.
Facebook is not stupid enough to tell a bald faced lie to advertisers and hope that nobody finds out. I would be confident in saying that Facebook has very specific metrics that they present. They aren't going to shrug their shoulders and say "eh, we have about X users". They be careful to say something like "we serve X requests per day" or "we have X numbers of engagements from people". They will make specific and factually correct statements. Those statements may very well be crafted to mislead people, but Facebook has every incentive to make sure they are not actually lying. It's not like they are starved for people buying ad space.
In general, I think it's pretty arogant and ignorant to claim you have a perfect understanding of a situation that you have no actual insight into. It is always safe to assume that there are better informed people who have thought a lot more about any given issue than you have.