There is generally a lock-out period to make sure people are buying the company's shares, not all the employees' shares.

Can employees still sell their shares on secondmarket now that FB is public?

They might be able to, but there is no real reason to buy on a second market now, so the market would be illiquid. If the employee would want to sell now for fear of a devaluation during the lock out period they could buy put options at the current valuation, haven't checked the price, but those options can't be too expensive right now.

Employees are almost certainly restricted to trading windows independent of any lockout provision. Given the lockout, it's unlikely that there is currently an open trading window. Any insider trading options outside of that window is likely to run afoul of the SEC.

There is a 6 month lockup period.

3 months in this case[1]

[1] http://www.inc.com/eric-markowitz/facebook-going-public-inve..., Ctrl+F for "90 days"

Gotta dump those shares before the tax code changes next year.

Then Facebook employees better hope the stock price doesn't fall to half of what it is now in 3 months.

I guess we'll be seeing a lot of 2 week notices in 6 months..

People call it "calling in rich". http://avc.blogs.com/a_vc/2006/07/vc_clich_of_the_3.html