Shares of LinkedIn (LNKD) started trading on the NYSE today. It will go down as one of the most successful IPOs in the social networking space. Shares popped up over 140%, an hour after trading began.

Shares were initially priced in the $32 to $35 range but prices were upped to  $45, valuing LinkedIn at about $4.11 billion. The company sold 7.84 million shares at $45 each to net $352 million.

LinkedIn operates a website where professionals connect with one another. They use the site to search for jobs, recruit employees, and find industry experts. While users can create personal profiles for free, paid subscriptions were introduced in 2005, giving recruiters more access to candidates and providing professionals ways to communicate with one another. The company gets 70 percent of revenue from business subscriptions, a model that’s similar to Salesforce.com (CRM).

The LinkedIn IPO is being watched carefully by investors in Facebook, Twitter, Zynga, and Groupon to gauge investor appetite for social networking stocks.

“If LinkedIn goes out and doesn’t trade well, that could present a problem marketing other social media companies going forward,” said Yvan-Claude Pierre, a corporate lawyer at DLA Piper in New York who has represented companies that are going public.

However, many think that the shares are too over-valued.

“I wouldn’t touch the stock, I wouldn’t own it, not at $45, not at $43,” said Eric Jackson, managing member at hedge fund Ironfire Capital. He might consider buying it at $25 along way down from its high of $122 today.

“The valuation for LinkedIn is rich,” said Michael Moe, chief investment officer of GSV Capital Management in Woodside, California, in a televised interview yesterday with Bloomberg West. “To earn the valuation, it has to continue to grow very, very fast.”

Last year LinkedIn reported revenues of $243.1 million and posted net income for common shareholders of $3.4 million.  The company disclosed to the SEC that it does not expect to be profitable in 2011.

To get the latest news on LinkedIn visit the LinkedIn Channel.