Having worked at Netscape in the Internet bubble, I have a natural pessimism when it comes to Internet pure play stock valuations. Just because you could order a product on the Internet doesn’t make the market for that product exponentially larger. It also becomes hard to monetize users that don’t use your service to actually buy things. Look back to AOL to realize that it’s hard to make money providing free services (like AIM) regardless of how many users there are.
Facebook seems to me to be a classic example of ‘Irrational Exuberance’. Duff McDonald at Fortune Magazine also shares my perspective and has a pretty good writeup on his reasons the discussed valuations of Facebook are interesting but probably not realistic.