Andreesen is a board member, so under Facebook's insider-trading policy he is required to conduct sales under a Rule 10b5-1 plan [1]. From FB's most recent annual proxy statement [2]: "Certain of our directors and executive officers have adopted written plans, known as Rule 10b5-1 plans, in which they will contract with a broker to buy or sell shares of our common stock on a periodic basis. Under a Rule 10b5-1 plan, a broker executes trades pursuant to parameters established by the director or executive officer when entering into the plan, without further direction from them. The director or executive officer may amend or terminate the plan in specified circumstances. Our executive officers and directors are required to conduct all purchase or sale transactions under a Rule 10b5-1 plan." (Emphasis added.)

"A company making such disclosure [that an insider has adopted a 10b5-1 plan] generally will disclose the existence of a plan but not the specific details." [3]

[1] Form 4 reporting Andreesen's sale: http://investor.fb.com/secfiling.cfm?filingID=1127602-15-304...

[2] http://edgar.sec.gov/Archives/edgar/data/1326801/00013268011...

[3] http://media.mofo.com/files/Uploads/Images/FAQ10b51.pdf ("MoFo" is Morrison & Foerster, a leading Bay-Area BigLaw firm.)