1)It's a good way to stay connected with family/friends, it's gotten to be alot more convienent than e-mail. My mom and brother/sisters are on Facebook and now that I'm out of the house it's a great way to stay connected. Also helps you find out friends birthdays, phone numbers, etc..
2)Photo storage- It's a great place to archive photos, has all the privacy features and tagging options of Flickr
3)Facebook Chat- A good way to chat with your friends, doesn't have all the functionality of AIM or ICQ but a nice addition to the other reasons.
4)Everybodys doing it- Not just all your friends, but companies, bands, movies, everyone is starting to have a facebook presence. Companies give out great coupons exclusively to facebook fans. You can see exclusive movie trailers.