Because most people I know use it. I keep up with their news, especially friends abroad and kids photos etc. It also helps to find old friends. Just seen someone 3 times in the last three weeks who I hadn't seen for a decade before that because of facebook.
I don't post much and I delete posts after a week.
I understand it's a business providing a service and that they make money by selling data regarding you and me. I just don't want my data on their shelves to sell.