It's a $70bn ad network. All that revenue comes from highly liquid ad budgets. It's naive to conclude that fb ads "don't work" from anecdote... Not working most of the time is the norm for advertising, but the scale comes from the cases where it does work.
I will be honest and say that the prefix was assumed to be understood without having to be written, but I understand how without it my statement could be seen as something else (and rather obviously wrong).
Anyway... this is just not the case here. The advertisers that pour budgets into fb aren't suckers. They're highly roi-sensitive and data cognizant.
All advertising fails much of the time, but when it doesn't it scales.
How do you know? Facebook obviously carries ads from huge brands, but also has a very long tail of smaller businesses and other advertisers. While it's reasonable to assume that whoever is spending a few million here and there on behalf of some major retail enterprise is watching the numbers pretty closely, I see no reason to assume that a thousand small businesses who are spending a few thousand here and there are doing the same. It's also worth noting that several things Facebook has done in the past have prompted reports of Major Brand X pulling $Y million in ad spend, so apparently the people who do have the sophisticated ROI monitoring don't always like what they see.