Will Fad-book still be popular in "years"? Look at MySpace...
This time last year, MySpace was the biggest thing in social networking. Now it's Facebook who're at the peak of the game.
The teenage crowd which gives the generalised social networks their momentum is notoriously fickle and next year's generation never wants to be seen using the same sites that their older siblings use.
Unless Zuckerberg has a plan to blackmail users to stay with the ridiculous amount of personal data Facebook harvests, in 12-18 months Facebook will be another forgotten niche site (along with ICQ, Orkut, HotOrNot.com and now MySpace) and it's valuation will plunge.
IPO in a few years? More like buyout at bargain basement rates by an ageing dinosaur wanting to appear "down with the kids" but only managing to buy up companies in mid shark-jump and knock them out of the air for good (ie. AOL and Mirabilis, AOL and Nullsoft, AOL and.. er).