A typo (in all likelihood) for either SSNs or "Social Numbers".

Could also be SMS (phone #)

Very unlikely.

Why would Facebook collect SSNs?

Like he said -- "People just submitted it".

But... why? When? How? I joined Facebook in 2006 when it opened to the public and it never asked for my social security number. Did they used to ask that prior to 2006? I mean, "people just submitted it" only works if it was asked for. Unless we're presuming people posted their SSNs on their wall for their friends to see.

I really doubt that's the case.

This isn't about Facebook though, it's about some prototype he created earlier. I'd say he probably had SSNs because lots of universities used SSNs as student IDs before it became universally clear why that was a terrible idea, and he probably required your student ID number to use the service.

Yes, it was from an earlier prototype of FB. But the bigger point is that it was Zuck who was saying that. And it tells you everything you need to his real attitude toward your privacy -- no matter what his "policies" say.

As well as attitude towards his users as human beings generally.