Happens to me all the time. Assume the ids are drawn straight from a database that has been set up to use integers for ids. Facebook was originally built with PHP and MySql. This happens to me: I start a project, create a few user accounts, and then delete them all. Then I create my real account. So I end up with an id of 5 or 6.
The earlier accounts look like they were deleted:
http://graph.facebook.com/3