Influential venture capital outfit Andreessen Horowitz has invested $80 million in microblogging phenomenon Twitter, adding to its growing portfolio of social Web names.

The venture capital firm bought Twitter stock on secondary markets for private firms a few weeks ago, joining Kleiner Perkins and other high-profile investors in a four-year-old website that allows users to send 140-character messages, or tweets.

A spokeswoman confirmed the transaction on Wednesday but would not comment on what sort of valuation it gave the hot start-up. Twitter joins Facebook, Foursquare, Zynga, Skype and Groupon in the venture capital house's portfolio.

The investment Netscape co-founder Marc Andreessen's venture capital firm follows a $200 million financing round in December led by Kleiner Perkins Caufield & Byers, which valued Twitter at $3.7 billion.

Twitter, which had 175 million users as of September, is among a new crop of fast-growing Internet social networking services. Others include gaming outfit Zynga and Facebook, the world's No. 1 Internet social networking company with a current valuation of about $50 billion.

Twitter chief executive Dick Costolo, the architect of its new advertising efforts, is said to be prioritizing money-making ahead of an IPO in coming years, but the company has yet to offer any public glimpses into its finances.

(Reporting by Alexei Oreskovic)

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