Microsoft published the biannual transparency report The number of US foreign intelligence surveillance requests more than doubled. This is the highest number of request Microsoft has listed since 2011, when it began tracking such government surveillance orders. Microsoft also released for the first time a national security letter (NSL), a sort of warrantless surveillance order used by the FBI to access data of a customer of a company. Microsoft isnt the unique company that disclosed an NSL, Twitter and Yahoo in the recent months made the same under a transparency measure of the USA Freedom Act.”]
Source: http://securityaffairs.co/wordpress/58063/digital-id/microsoft-biannual-transparency-report.html