I've only got 6 so far, I think. The only criteria, really, is that it has to be clever in some way and make me laugh on a consistent basis.
evilbrennan has the distinction (so far) of being the only person on my friends list to be added to my little collection (wait, not like that). The rest seem to be from the more remote "friends of friends" category, or even more spy-ey, the "friends of friends of friends" category.
UPDATE: Freakit...I never get that LJ user code right on the first try. >stomps foot