If Facebook will have a $100 billion market cap, and likely higher soon after the IPO, then it's certainly reasonable to think the Linked in will have a $15 billion market cap. That implies a $150 target for LNKD. I think $120 is certainly possible by the day of the Facebook IPO.
I was a Linkedin stock owner until yesterday. I don't feel comfortable either way with this stock. It obviously has a price that is hard to justify from a valuation standpoint, but it seems to behave like a Ouija board. Who know what direction the stock will take. It was up yesterday when the market was down big time and down today when the market was up fairly big. The big institutions control the Quija board.
What makes you think that if facebook $100b, then linkedin $15b. People spend 20 times less time on linkedin then on facebook. Multiply 20 by 6, 6 is the difference in users 150million/900million. So you are getting120 times difference. Facebook 75/100b, linkedin 650-850million.