Facebook stock is down nearly 3% following the news that Marc Andreessen sold most of his stake in the social network

Marc Andreessen and Facebook's Sheryl Sandberg

Facebook shares are off about 2.7% on Monday following news that board member Marc Andreessen has sold about three quarters of his stake in the company over the past two weeks.

Andreessen sold 1.5 million Facebook shares, valued around $160 million, through pre-arranged trading plans beginning on October 30, according to calculations by Recode, which first reported the share sales.

Andreessen's venture capital firm Andreessen Horowitz was one of Facebook's backers before it went public in 2012. So the fact that it is cashing out is not completely unusual - VC firms often sell shares of their portfolio companies during or after IPOs in order to give their limited partners a return.

But the amount of shares that Andreessen is selling in such a short period of time, and the fact that he sits on Facebook's board, appears to be troubling investors.

The share sales come a few weeks after Facebook reported a blockbuster third quarter, handily beating Wall Street financial targets and sending its stock to a new all-time high.

Facebook shares were down 2.7%, or $2.75, at $101.20 in midday trading on Monday.

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