Not just fickleness...
...but laziness. Let's face it, keeping your myspace/facebook/blog/whuteva up to date takes time and effort.
Forget boredom - there just isn't that much happening in my life that I can be bothered writing it all out for ten million strangers to read.
11:42: had a shit. It was big and stinky.
Yup, that was the highlight of my day. Hope you enjoyed it.
Games keep us going because they change. WoW and other mmporgs hold interest (although, how many of the people playing a year ago are playing now? I'll bet the actual turnover is fairly high) because they aren't static. Also, you can kill things.
It doesn't take long to fill in your details on the social networking sites. After that it's just the same thing day after day - there's no change. And there's no change because we're lazy - we're the ones generating the change. That takes too much effort. When something takes constant, driving effort we rebel, and push it away.
It's the same reason television hasn't completely lost out to the internet - we just can't constantly interact. Sometimes we just want to sit back and be entertained without the need to engage.
Folk who make social networking sites are themselves very social people. But as gregarious as humanity is, we still prefer to have our own space - that's why we build houses with opaque walls and get annoyed when someone peeks in our windows. Being social is not a 24/7 thing.
Static sites, laziness, privacy - add it all up. That's why these sites will eventually lapse into obscurity.
Still not convinced? Ask yourself this: how many of your friends had a blog two years ago? How many of them are actively maintaining them now?