Who Has Option to Sell Facebook After Lock-Up?

It’s a moment of truth for Facebook as shareholders evaluate whether to sell 271 million shares Thursday, as the first of several lockup period expires. That adds up to ten percent of shares outstanding.

The big question: Will a flood of shares send the stock even lower? But with shares off 45 percent since its May IPO, some say it may not have much further to fall. (Related: Why Facebook Shares Won't Fall After the Lock-Up.)

So who has the option to sell on Thursday?

The list includes early investor Peter Thiel: He sold over 16 million shares at Facebook’s IPO, and recent SEC filings reveal that he converted some 9 million shares to Class A shares from Class B shares, giving him more flexibility to sell. (Related: These Investors Sold Stock in Facebook IPO.)

Venture Capital firms Accel, Elevation Partners, Greylock, and Tiger Global are also on the list. VCs often sell soon after companies they’ve invested in start trading, or they might opt to distribute shares to their partners, to give them the choice of whether or not to sell.

Microsoft will be able to sell, but the tech giant is unlikely to, because its investment in Facebook is a strategic one — the two companies have an ad partnership. Goldman Sachs and Russian investment firm DST are also on the list of potential sellers — the final S-1 filing before Facebook’s IPO reveals that DST has the option of selling 75 million shares on Thursday.

Both Goldman and DST bought into Facebookrelatively late — at much higher price than many of those VCs — so the stock’s decline means they’re sitting on much lower profits than they were at its $104 billion IPO valuation. We’ll see whether they choose to cut their losses and run, or whether they see more upside. (Related: )

Thursday’s test is just one of many for Facebook over the next nine months — those 271 million shares are just a fraction of the roughly two billion that will be freed up between now and May.

The biggest risk to the stock is likely to come on November 14, when over a billion shares could hit the market. Still, the decisions early investors make Thursday will be a crucial indicator of confidence in the company.

—By CNBC's Julia Boorstin
@JBoorstin