Facebook: Why is it so important to people?

I can’t stand people who take things like Facebook seriously.
I’ve noticed on my friends news feed, people going in and out of relationships, sometimes weekly. Apparently breaking up and getting back together, and it HAS to be put on Facebook instantly.
And people who take who you are friends with on social networking sites, as to actually mean something. Not everyone I am friends with is someone I am close to. They are people I’ve worked with, went to school with, or met through friends. I don’t have a close relationship with all of them. But yes, I do use discretion when adding people, because I don’t want people I do not know on my friends list and seeing my personal information.
Why is adding someone you your friends list a big deal? Why do people take Facebook so seriously?