In summary then
FaceBook takes money from scammers in return for putting their scam adverts up on their platform.
The scammers, naturally, hope to make money from their scams – that's why they pay FB in the first place.
The scammers' activity is, presumably, illegal.
FaceBook's activity, however, is not at all or even in the slightest little bit illegal. Oh no. Definitely not illegal.
FaceBook's position is:
(1) No we can't possibly be held liable for any scams or other illegal activity that may be associated with ads or other content on our platform. Those ads are of course absolutely nothing to do with us because, like all content on our platform, we don't originate it and what's more we have no relationship with the people who do originate it either – well – other than the part where we take their money in return for putting their content up – but after all we're not a fucking charity are we.
(2) Also, no of course we can't possibly vet all adverts in advance. Are you some sort of technology illiterate moron? Do you have any idea how expensive that would be for us?
(3) Tell you what – if some of you dumb fucks want to get together in an amateur kind of after-work pond-life-association or whatever and tell us when you think you've spotted something horrid on our platform then knock yourselves out (just so long as we don't have to pay for it in any kind of meaningfully ongoing statutory or profit sapping kind of way). We might even eventually get round to taking some of the stuff you point out down – then again we might not. The main thing here is to keep it all nice and non-binding and really really vague so no one is ever quite sure if we actually have to take anything down or not, certainly not in any kind of if-we-don't-take-it-down-we-go-to-prison kind of way. This way we can make it look like we're doing a massive favour to the community (whatever the fuck that is). But then we're so giving. So virtuous. So so virtuous.