As an advertiser, I am pretty certain this is not the quite case in (current) reality. A large part of FB's proposition for more money from us includes:
A) Pay more for increased reach and engagement.
B) Our traffic isn't decreasing (despite outside reports/indications to the contrary) and you would be missing a massive and engaged audience if you didn't spend with FB.
This combined with the fact that a _lot_ of ad-spend isn't directly attributable to conversions (often by design), means that more "activity" whether its fake or not, drives up ad-revenue for FB.
You see the same issue occur with other publishers by the way. -It is not uncommon for a publisher (or other related party) to purchase a swarm of fake bot traffic to boost impression and engagement numbers of an ad buy they've sold. -Advertisers un-aware of how much of the traffic to their ads are bots vs legitimate humans (read: "publishers stealing money from advertisers") is a major problem for advertisers, but the bigger the publisher, the harder it is to 'not' be on their platform too. (and FB is _very_ big)