Look up the key internet companies that are located in Greece. The email address used to sign up would of course only be eligible if it came from someone using an email from said Greek provider. Then include some IP checks in just as a backup safeguard (which of course is not infallible).
I think it could work, perhaps not perfectly, but it could be something to consider.
I think it would be easier to abuse email accounts / servers / IP checks, than it will be to fool social network auth at this point.
We know that facebook and google at least *wants* to verify uniqueness.. it may be a hard task and they may not have solved it 100%, but they've set teams within their companies to tackle this problem already. After all, it is their business on the line.
IP checks would be very easy to fool, so that could just be added in as a backup extra type of thing.
As for an email account, I am not sure, but is it difficult to verify that an email only works with a specific provider? Of course they can be spoofed, but I assumed it wouldn't be that hard for a dev to figure out who is spoofing, and who isn't?
And when I say email accounts from a Greek provider, I don't mean freebie email signup things. I mean the sort that are linked to paying internet accounts. I expect most accounts get several emails, so it's not 100% accurate ... then again, there may be several people per household on average anyway.
I may also be wrong, but I also think people might try to abuse facebook accts more than trying to fake emails from some greek internet provider than nobody outside of Greece has ever heard of before.
Anyway, just an idea.
