Facebook shares closed down more than 6 percent Thursday as the lockup expired on 271 million shares. Volume spiked, with over a hundred million shares exchanged hands by mid-day. That’s up over 160 percent from the ten-day average.

Now we’re watching to see if any of the insiders are selling — if they sell Thursday, SEC filings aren’t due until the end of day Monday. (Related: Accel Unloads Facebook on Its Own Investors: Sources ).

With the stock’s decline, its market cap is now below $50 billion, the valuation at which Goldman Sachs bought in. And Microsoft , which invested at a $15 billion valuation, is also sitting on much smaller gains than it was going into the IPO. (Related: Can Facebook Unlock Value? )

Other shareholders who can now sell stock today include Russian investment firm DST, and early investor Peter Thiel, who recently converted 9 million class B shares into Class A shares, giving him more flexibility to sell. (Related: Who Has Option to Sell Facebook After Lock-Up? )

A list of venture capital firms are also on the list: Elevation Partners, Greylock Partners, and Tiger Global.

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