Why do you think facebook will die away? Its strongly ingrained in most social communities (especially youth ones) and there is a strong social incentive to it.
Everything in this world (even you and I, my friend) are slowly dying. For some things, it takes a while, for some others it may not. The one thing we all have in common though, is that we also all have a peak in our life cycle. Do you believe that Facebook's peak is yet to come, or do you think it's happened already?
The question you have to ask yourself is what has made Facebook rise? Will it continue to do so? Is it's business model sustainable forever? Of course not, no business model is.
That being said, I agree with with Grahambo on this. If you watched the "Social Network" you will remember that Zuckerburg was adamantly against advertising, he said the value of Facebook was that it was "cool". He was totally right on that and he has made billions by having the "coolest" internet craze of possibly all time.
But what now? Their future revenue growth will be based in advertising and pay-per-click. They are sacrificing what makes them cool to cash out. Facebook (again, in my opinion) has peaked in it's business' life cycle. I don't think that it will be a steady drop off until it's death, but it will die.
Page me on my beeper if you want more info on integrated technology in our lives. I cant live without that damn beeper.