Sometime earlier this year, a provider of communication services in the United States – perhaps a phone company, perhaps Twitter – got a letter from the FBI demanding it turn over information on one, or possibly even hundreds, of its customers. The letter instructed the company to never disclose the existence of the demand to anyone – in particular, the target of the investigation.
This sort of letter is not uncommon post-9/11 and with the passage of the U.S. Patriot Act, which gave the FBI increased authority to issue so-called National Security Letters (NSLs). In 2010, the FBI sent more than 24,000 NSLs to ISPs and other companies, seeking information on more than 14,000 individuals in the U.S.
Full article in Wired. Stats towards the bottom.