Facebook won't be "killed". It probably won't disappear in an instant. But it may most likely fade away in time due to a confluence of many factors, not just one: 1. early adopters growing up, 2. decreasing marginal utility, 3. failure to directly align revenue with consumers, 4. lack of vision, 5. corporatism, 6. solipsism, 7. something better coming along.
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)
IF (big if) Facebook's profitability is only illusionary[1], then Facebook could die in an instant. If Facebook's profits only come through churning advertisers, FB could die if the growth that pulled in those advertisers stopped.
It costs real money to keep Facebook's servers running and Facebook's employees working. If a market shift revealed Facebook as a money-sink "as far as the eye can see", then the folks other than Zuckerberg who control that money will want to use it for something else.
One might ask what magic does FB really have for profitability that Myspace didn't have? I'd like to hear how this magic will come from more users in and of itself.
See: "How the Mighty Fall" by Jim Collins (http://www.amazon.com/How-Mighty-Fall-Companies-Never/dp/097...).