Even if you could prove that Facebook is filling the gaps between new users and churned users by creating fake accounts, you also have to draw a line between those fake accounts and the company's bottom line.
Fake accounts don't buy products, so if advertisers are making decisions off of CAC, numbers potentially inflated by fake accounts (reach, clicks, engagement, etc.) are secondary.
Facebook's ad power comes from their egregious data collection and lack of privacy concerns. The more they know about you, the more relevant your ads will be and the more likely someone seeing your ad will be to buy. And if advertisers can draw a line that says "if I spend $50 on Facebook ads, I'll increase my bottom-line line $100", they'll spend money until it's no longer profitable to do so.
That said, it's not like all Facebook advertisers are acting rationally in that manner. I don't know what % of Facebook revenue to coming from unsophisticated advertisers, so it's possible that they could be making a lot of money from people focusing on fuzzy metrics like reach.