Apparently, though, when you get over 500 on your Friends list, the Userinfo page puts a special link next to 'Friends' that takes you to a directory page, supposedly more suited for viewing such a list. See gigglecam's user info page for an example.
However, clicking on this link produces an error:
[Error: Undefined subroutine &BMLCodeBlock::connect_db called at (eval 13) line 20. ]
Not a big deal, since users with these large lists report still being able to manage their list; it's just not viewable by clicking on the link.