During the second half of 2012, Twitter received 1,009 requests for user information on 1,433 accounts. 815 of those requests (81%) were from the U.S. and related to information on 1,145 user accounts. Sixty percent of the U.S. requests were by subpoena, 11% involved a court order, 19% involved a search warrant, and 10% were reported as “other.”
You can read more of their report here. Do follow the link to additional info on the U.S. requests, where Twitter also provides stats on the breakdown of cases where users were notified or not notified.