Facebook was very valuable to me when I was in college because it was like an addressbook for essentially all of my classmates. So, if I ever forgot an assignment, I could just look up the class and find fellow student to IM, email, or otherwise contact. Moreover, it allowed me to meet new students when I transfered from the Univ. of South Dakota to American University in DC through the "Groups" features. So, I was able to find other gamers, other Arrested Development fans, etc.
Now that I am not in college, I use Facebook to keep in contact with fellow alum, as well as people that have not graduated. Email addresses, snail mail addresses, IM contacts... they all change, but with Facebook, I am able to update all of my friends without much trouble.
All that said, I may check my account once/month.