There's an interesting analysis of which web company is worth the most per user at http://neptune.observer.com/node/138871 - LinkedIn ranks 3rd on their list of 9 arbitrary companies:
Groupon - $210
Facebook - $105
LinkedIn - $33.33
Zynga - $25.58
Twitter - $18.05
Foursquare - $20
Tumblr - $15
Skype - $1.56