Some prefixes from my friends lists:

100k = Columbia 200k = Stanford 300k = Yale 500k = Dartmouth 600k = Penn 1000k = Brown

Cornell = 400k, MIT = 700k, NYU = 800k, BU = 900k, Princeton = 1100k, UC Berkley = 1200k, Tulane = 1300k, Georgetown = 1400k, UVA = 1500k, and then I got bored.

You can keep plugging away IDs into http://facebook.com/profile.php?id=[id]