Google publicly acknowledges that the FBI tracks some its customers' activity

"…it is unlawful for any record-keeper to disclose it has received a so-called National Security Letter. But under a deal brokered with the President Barack Obama administration, Google on Tuesday published a 'range' of times it received National Security Letters demanding it divulge account information to the authorities without warrants." …

See the report here: Google Transparency Report - User Data Requests

"National Security Letters are a powerful tool because they do not require court approval, and they come with a built-in gag order, preventing recipients from disclosing to anyone that they have even received an NSL. An FBI agent looking into a possible anti-terrorism case can self-issue an NSL to a credit bureau, ISP or phone company with only the sign-off of the special agent in charge of their office."

Google Says the FBI Is Secretly Spying on Some of Its Customers (Wired)