The lockup expires in May for the rest of the 474 million shares, and the market reaction at that time will be very interesting to watch.
Here's a link to a WSJ Digits blog post on the lockup:
http://blogs.wsj.com/digits/2014/02/13/twitters-first-lockup-expires-on-saturday/
Here's a link to Twitter's IPO prospectus (to the Underwriting section that discusses the lockup):
http://www.sec.gov/Archives/edgar/data/1418091/000119312513431301/d564001d424b4.htm#toc564001_21