Interestingly, in Zynga's case, the lock-up period is 165 days rather than the typical 180 days.
My take on this is that the filing is to allow certain venture capital investors to sell their shares in a managed process. Zynga's venture capital investors include Kleiner Perkins Caufield & Byers, Institutional Venture Partners, Foundry Group, Avalon Ventures and Union Square Ventures.
Here's Zynga's SEC filing for the secondary offering: http://www.sec.gov/Archives/edgar/data/1439404/000119312512113668/d312579ds1.htm
Here's Zynga's prospectus for it's IPO in December (information on the 165 day lock-up is found in the underwriting section): http://www.sec.gov/Archives/edgar/data/1439404/000119312511343682/d198836d424b4.htm
Here's a link to the SEC's website discussing IPO lock-up agreements:
Here are some press reports discussing the offering:
Here are links to my prior posts on Zynga: