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They haven't always been globally sequential though. It used to be of the form AAAABBBBB where AAAA was the school ID and BBBBB was the user ID within that school.

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.

I'm pretty sure the IDs aren't generated sequentially. Sometime in the last year or so, they started prepending '100000' onto each new user ID. Also, the rest of the new ID format is at least 11 digits long. I'm not sure how they did it in 2004, though.

Facebook user ids are not generated sequentially.