A national security letter
is used by US government agencies such as the FBI and NSA
to demand access to data from companies – who are forbidden from revealing that they have been served such a request.
publish "transparency reports" detailing how many government requests they receive for user data in various countries, but those for the US do not include Fisa requests or other NSL demands. Facebook has not so far published a transparency report.
Microsoft said: "Our recent report went as far as we legally could and the government should take action to allow companies to provide additional transparency."