Linkedin is the first publicly traded success story of the social networking era, valued at $8.4 billion, a  547 times the company's 2010 earnings, and it keeps attracting new members. (See its CEO Jeff Weiner in the above's video)

But what strucks me more than this whopping valuation are its new revenue sources beyond advertising and subscriptions. For example, recruiting firms pay Linkedin an average of $8,000 a year for each employee who uses its avanced tools to search through profiles on the site. Hiring solutions accounted for 42 percent of revenue in 2010.

In this fied, a few companies are using Facebook's network to take on Linkedin directly.

• San Francisco's BranchOut gives Facebook users a way to discover job opportunities through their friends
   
• Monster has launched BeKnown, a Facebook app for professional contacts.