San Francisco: Professional networking site LinkedIn raised $53 million from investors in a fourth round funding, giving the company an estimated valuation of just over $1 billion, the company Chief Executive Dan Nye said.

Private equity firm Bain Capital's Bain Capital Ventures led the investment round, and LinkedIn's existing investors, venture capital firms Sequoia Capital, Bessemer Venture Partners and Greylock Partners also joined, the company said.

According to Jeffrey Glass, Venture Partner at Bain Capital Ventures, the $1 billion valuation is a "big number if you think about it in absolute terms." But Bain felt comfortable with its calculation given LinkedIn's popularity and the growth opportunities that lie ahead, Glass said. "We feel like we'll make a nice set of multiples on our money."

A handful of Web 2.0 start-ups, providing people innovative ways to interact online, have recently secured high valuations. Last year, Microsoft invested $240 million in Facebook, valuing the social network at $15 billion. This January, Slide, a start-up that lets people create photographic slide shows for social networks, raised $50 million from institutional investors in a round that valued the company at $500 million.

LinkedIn, which is five years old, has so far raised more than $80 million, including the latest round, Nye said in an interview last week. "LinkedIn is growing like gangbusters and it is clearly becoming a critical business tool," he said. Nye said he expects the network, which currently has 23 million members and is adding 1.2 million new members every month, to have more than 30 million people by the end of 2008.