I don't know this for a fact, but if I were FB I think I'd be fanatical about verifying real users to prevent people setting up social media PBNs. A holy grail of grey-hat SEO nowadays would be to control large networks of interlinked fake social media accounts, which could be used to promote content artificially. This kind of spam is potentially hard to detect since the networks could be very large and appear organic (to the point, theoretically, of having AI-driven "users" behind each one), so the first line of defence is to identify fake user accounts.