That is 500 ACTIVE they say. I'd guess they have close to 600million accounts, maybe 650m.
I also find it hard to believe that 75%+ of people who have -ever signed up for Facebook- would be active in the past month.
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[1] Obviously not all of these are real, many could have been deleted, etc. But we're talking about gross number of accounts, not humans.
I don't know where they started the current sequential IDs from, but it was probably at least 100,000,000, which would have allowed for 3 digit school IDs.
I could be totally wrong though.