Facebook board member Marc Andreessen has sold more than 1.5 million shares of Facebook stock worth roughly $160 million in a pre-arranged trading plan that commenced on 30 October, according to US Securities and Exchange Commission filings.

Andreessen has left over 73% of his total ownership in the social networking site, along with 90% of his class-A shares, which don't bear any voting rights.

Although he still reportedly owns nearly 378,000 shares of Class B stock, voting rights for Facebook don't count as CEO Mark Zuckerberg dominates 60% of the board's voting power.

Andreessen's venture capital firm "Andreessen Horowitz" was one of Facebook's backup companies before the social networking company went public in 2012.

The sales took place during the time Facebook reported its quarterly earnings on 4 November. The social networking site beat estimates by analysts, accomplishing a new milestone of reaching one billion people every day. The shares had increased almost 6% from the end of October to 12 November.

However, after Andreessen sold his stakes, midday trading on Monday, 16 November, showed Facebook shares went down 2.7% or $2.75, at $101.20, reported Business Insider.

Andreessen's shares were sold through the "Rule 10b5-1" trading plan that says the shares were sold on a particular schedule to reduce any issues related to insider trading.

However, there is also a possibility that the plan was made in order to carry out trades when the stock touches a particular price at a particular time, or permit the owners to assign a broker they see fit to purchase or sell, given they don't hold any inside information, reported Recode.

The sale is particularly significant because firstly, it involves lots of money, and secondly, the pace and quantity at which Andreessen has been offloading his shares in the past couple of weeks is uncommon among executives of any company.

Generally, pre-planned trading takes places at regular and steady gaps.

A spokersperson for Andreessen had reportedly declined to comment on this issue.