NEW YORK, Nov 10 (Reuters) - Facebook Inc director Marc Andreessen's venture-capital firm sold a third of its stake in the world's No. 1 online social network, according to a recent regulatory filing.

Andreessen Horowitz sold 2.28 million shares on Nov. 6 at about $49 to $50 a share, according to a filing with the U.S. Securities and Exchange Commission on Friday. It still holds 4.57 million shares in Facebook.

Andreessen Horowitz spokeswoman Margit Wennmachers said on Sunday that the firm's investment thesis was successful. Still, she said they are big believers in Facebook, which is why they are holding on to big portion of their shares.

Facebook shares closed at $47.53, down 3 cents on Friday.

Marc Andreessen helped start browser company Netscape, which was acquired by AOL Inc in 1999. Andreessen Horowitz has invested in some of the country's hottest start-ups, including Facebook and online bulletin board Pinterest.