I'm in ad tech, and believe me: FB advertising works. The vast majority of their ad revenue is coming from experienced ad buyers that are spending immense amounts on direct response campaigns, not branding.

If you want to see how things work, start a small ad campaign yourself of FB. It's all about ROI, attribution, cost per action, super detailed targeting, etc. It's the opposite of "throwing money at it hoping that it'll work", unlike offline advertising or even traditional display ads.