The barrier to creating an unlimited number of facebook accounts is a half step above creating an unlimited number of gmail or hotmail accounts.

No, the whole point of connecting with facebook is to focus on the UX of making electronic payments. Who's going to remember their coffee date's wallet is 1BeX1pVAjD94hNs6C5d2dB3TRnLx53dXg3? QR codes are cute, but haven't quite hit the mainstream. It's a lot easier to say "Ben Bitdiddle" and have Ben's facebook picture pop up.

The network effects of scanning your entire social graph (and the data contained within) doesn't hurt either. Works fantastically for Venmo.

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...

I wonder how they'll prevent this with the other authorization options which apparently are on their way

At minimum, N auth options mean an easy N*5000 stellars per unsophisticated user.

(I can create a FB-linked account today, and say an SMS-linked account tomorrow, and then merge the two).

Why of course? Since when has the existence/linking of an FB account to some other service been the only way to get more free Stellars? And who said, a Stellar has a real value other than being pre-mined bits and bytes?

You still can if you are willing to contact the people that do the Facebook Ad manipulation. ;)

I wonder if its to make it harder for us to all start selling drugs to each other.