Much more than users apparently. It looks like everyting is thrown in here:
Anyone knows why they are skipping IDs? Why didn't he pick id=1?
Re:Facebook Graph API robots.txt
by oscardelben on Friday April 23, 2010 @04:19AM
(#1287582)
Perhaps those were just tests, and databases use incremental index but don't care if you delete a record.
you can set the userid as parameter and you get Mark Zuckerbergs Profile here : http://graph.facebook.com/4
Then you can simple count up to infinite to get the other profiles. The API has a Usage limit and blocks after a while