I believe we are in a bubble - however it is nothing like the one in 2000. Investors are expecting big things from many start-ups and looking at the amount invested in some I feel there will be some dissapointed VC's. Facebook is another story. Although it is simply an amazing business I cannot believe it is valued at $10 billion (based on MSN's alleged offer for 5%)
It is this type of valuation that creates the bubble. VC's not in on the game see this type of valuation and get keen to jump on... only to see more crazy valuations based on no real tangible revenue streams aside from promises and projections.
It is not often that a company gets an application so spot on that there is very little to add or take away. Facebook has certainly raised the bar on user friendly design and functionality. It wont be long before the staff are poached into other industries to design simply user friendly platforms for banking, ordering etc etc