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:
http://www.sec.gov/answers/lockup.htm
Here are some press reports discussing the offering:
Bloomberg:
http://www.bloomberg.com/news/2012-03-14/zynga-plans-400-million-share-sale-in-secondary-offering.html
CNET:
http://news.cnet.com/8301-13506_3-57397067-17/zynga-plans-secondary-offering-to-fend-off-stock-price-drop/
Here are links to my prior posts on Zynga:
http://www.allenlatta.com/1/post/2012/1/zynga-closes-above-ipo-price-finally.html
http://www.allenlatta.com/1/post/2011/12/zyngas-ipo-performance-an-analysis.html
http://www.allenlatta.com/1/post/2011/12/zyngas-ipo-on-track-for-next-week.html
http://www.allenlatta.com/1/post/2011/12/zynga-files-ipo-range.html