So offer a tool that can port everything over to the new site, I highly doubt teenagers and college grads care about "invested time" if they can easely move their shit over, young people is all about popularity and when facebook runs outta juice people will move on.
The reason I don't consider facebook long term value like google is because people don't need facebook, lets face it everyone uses google because they depend on it for search and when they are searching for something they want to buy, the keyword advertising systems works extremely well. Now look at facebook, it's a place for young people to snoop around and look at what their friends are doing, what part of this is a neccessity? does it increase productivity? the advertising placed on the site will have much narrower audience, this is why Yahoo failed they don't provide a vital function that is used by all age groups. Do I think facebook sucks? no, I think they've done a great job, however such a high valuation for such limited functions is just insanely stupid as it has no way to sustain value unless it's business strategy changes significantly.