Okay, it's been a while since I've messed with Facebook, but here's what I bet it is.
Each Facebook username has their own Username ID which can be harvested by different programs. So, say Facebooks last registered username got the User ID 29098903489008 then the one after him got 29098903489009. So, what this guy did, which is pretty clever, was created a program that ran through the USER IDs sending out friend requests to thousands of people a minute, most likely from thousands of accounts. This would be very successful too, since most users would be more inclined to accept a random friend, given that they don't have any friends yet.