Two things:

  1. In the paper Co-evolution of neocortex size, group size and language in humans, Robin Dunbar predicts that the maximum group size that humans can maintain as a cohesive social unit, based on the ratio of neocortex volume to brain volume, is 147.8 (100.2-231.1 at 95% confidence). Consulting the literature, he finds that there's a trimodal distribution of group sizes: bands at 30-50 people, tribes at 1000-2000, and an intermediate one. The mean size of the intermediate level group societies is 148.4.
  2. The AOL Instant Messenger servers impose a hard limit on the number of people you're allowed to put in your buddylist: 150.

(For more, and a better summary of Dunbar's paper, read The Magic of 150. Malcolm Gladwell also refers to the number 150 in his book The Tipping Point.)