Other sites like Pastebin use a sequential ID too, but the convert number to another base (like from base 10 to base 43). It's shorter too. (I haven't found the link to the related stackoverflow discussion)
I joined in 2005, when you still needed to be part of a school which had a Facebook network. At that time the ID for most users was made up of a network number (which was in the 4-digit range by the time my school was added) and then a 5-digit user code. So you could (and still can, given the right info) figure out the first person to join from your school, as well as when your school got a network relative to some others nearby.
You can still use this to find some mundane information about Facebook's early userbase. For example, the first non-Harvard schools added were, in order, Stanford, Yale, Cornell, Dartmouth, U.Penn, Columbia, etc., and that the first people added to most of those networks either grew up with or attended the same high school as Zuck.
Other sites like Pastebin use a sequential ID too, but the convert number to another base (like from base 10 to base 43). It's shorter too. (I haven't found the link to the related stackoverflow discussion)