Facebook gives you utility the first time you use it. It gets better as you figure it out and your friends join. So your marginal utility is increasing. But then after a while, all things being equal (i.e. Facebook makes no upgrades), your marginal utility must at some point start to decrease in accordance with the law of diminishing returns. It's like eating an ice cream. Maybe you have another ice cream and then another but at some point you're going to get sick. In Facebook's case, there's only so much Facebooking you can do before you start getting bored. Unless Facebook manage to keep launching new features, improving things to counteract this. And doing that is no easy task. As your user base grows, your software has to become even more and more addictive to compensate. I think there's more a problem with diminishing marginal utility with Facebook than there is with Amazon or Google or Apple or maybe even Twitter since the latter seem to serve a more direct, simpler, more basic need.
I think there's a big difference between being bored with your friends and being bored with Facebook. I'm bored of Facebook. It's sole use for me at this point is to track down people I can't find otherwise (e.g. people I haven't talked to in a long time)
