LinkedIn has just filed for IPO. In 7 years, they made a little money in 2006 and last year they made 10 mill on 161 mill revenue. They don't expect the company to be in the black for quite some time... LinkedIn could have more staying power because it is used mostly by professionals. But Facebook knows more info about you...
I think Facebook has a shelf life of 5 more years. Which is astounding for a social network. I think in the end though people will move on to the next big thing, after being tired of privacy infringements and constant spam. Just my two cents.