Other discussions here are saying that Stellar scans the Facebook account to check whether it looks like a spam account set up to exploit the giveaway. It rejects accounts that are newly registered, for instance.
I'd be surprised if that's true. I signed up with my fake FB account and it worked just fine. The account is a complete spammy mess so if they are attempting that, they are not doing a very good job. I would say more likely no new accounts allowed and that's it...