I think Facebook is overvalued. Reasons are :

1. Single profit channel, Facebook gain its most profit from advertisements display. On its right side bar, we can see many advertisements. But people who go to Facebook are those who want to connect with their firends. They will not put much time on clicking ads.
2. Weak in mobile market. Facebook's mobile platform development goes very slow, their mobile apps are still not so appealing even they have revised their apps a few times. The user experience is really bad. Now almost half of Facebook users are from mobile network.
3. Loss of employees. Since Facebook's IPO starts, it will create many new millionares. When these employees have money, they may choose to join new companies which may have bright future, such as Pinterest. Also some of them may start their own business or do some investments.
4. Users loss, although Facebook says they have over 9 millions users, among them, how many are active users? As for me, I seldomly log on to Facebook now. With many new social media coming out, such as Pinterest, SocialCam, we have more choices which are more attractive.

Unless Facebook can find a new business model which can utilize their huge user base, Facebook may face many difficulties in the future.