I've had this experience as well. I'd love an opportunity to chat with someone who runs these click farms for a living, so I can understand their motivations a little more clearly.

I can't wrap my head around why someone would run a bot farm to click on my FB ads. It's not like AdSense, where they're getting a cut. I've read a theory it's to hide the bot accounts from Facebook's bot-detectors. But if people like in your experience can easily ID the obviously fake accounts, FB can (using the same criteria you mentioned).

I get FB has moral hazard to allow bot clicks, because they get paid. But if they're so sophisticated as to require fake engagement on ads (which is why I care -- I'm getting charged!), wouldn't they also be monitoring the simple heuristic you provided?

This could be explained by Facebook requiring bots to make them money to not get banned, but I don't think there's sufficient evidence for such a conspiracy just yet.

In the end, all I care about is how much I put into FB ads, and how much directly attributable revenue that generates. The fraud might hide otherwise profitable campaigns (ones that don't break even), but frequently this isn't the primary concern. (My concerns are usually, "does this part of Lake Facebook have any fish I can catch?", followed by the step 2 of optimizing the campaign to get a positive ROI.