Even though I am 20 and in college and am friends with everyone I know in real life, everyone I knew growing up and from summer camps, and many of my internet friends as well, I rarely go on anymore. It used to be a daily activity.
What went wrong? I think it just became so popular, had so many people that it wasn't not interesting anymore. We all have a little hipster in us - once everyone you know and their mother and their grandmother is using something, it starts to lose its appeal.
Lastly, Facebook had too much change. They could have updated it once or twice with fairly big changes and been fine. I noticed that I hated hearing about facebook or going on facebook when they were rolling out new privacy agreements and rearranging the site layout once a week.
There is a big potential for anyone developing the next social networking site. Who knows what aspects will make it popular though.