Lockup doesn't expire until six months after the IPO. Employees and ex-employees have a while to wait until they can begin selling.

How many shares did the employees own?

Even if they all sold all of them, is it really enough to have a meaningful impact on the price of the stock with that much trading volume and that large of a market cap? Probably not!

If you count Zuckerberg as an employee, then I'm pretty sure he owns more shares than were issued in the IPO.

Even if you don't, a huge slab of the company is owned by current and former employees.

6 month lockup no?