Instant messaging is one of the main methods of communication among students behind cell phones and email. In 2008, AIM is by far the most popular IM program at Columbia. Now most people use Google Talk (because it's built into GMail), or Skype (for the cheap international calls, popular with international students).
A useful, though underused, IM client called Pidgin is available for Windows that allows communication with AIM, MSN, GoogleTalk, ICQ, and Yahoo! (with support for Skype in the works) all in a single program.