I guess I'll be the contrarian here. The real customers of FB (aka advertisers) that I've asked all claim to get their money's worth from their ad spends and obviously they keep going back to the well rather than switching to somewhere else. Fake users and bots can't be spending fake money. So even if the claims about the raw number of users are grossly exaggerated, the performance metrics which actually matter (eg conversions per ad dollar) can't be fake. Therefore the company can't just be an elaborate scam.