The USA National Security Agency has a tool called XKeyscore. This tool collects 'nearly everything a user does on the internet', including emails, social media activity and browsing history.
This means that potentially everything you have done, are doing or will do on the internet can be viewed by the American government - even your most intimate communications.
Analysts can search with no prior authorization through vast databases containing emails, online chats and the browsing histories of millions of individuals, according to documents provided by whistleblower Edward Snowden.
NSA says:
"XKeyscore is used as a part of NSA's lawful foreign signals intelligence collection system.... Every search by an NSA analyst is fully auditable, to ensure that they are proper and within the law... These types of programs allow us to collect the information that enables us to perform our missions successfully – to defend the nation and to protect US and allied troops abroad."
www.theguardian.com/world/2013/jul/31/nsa-top-secret-program-online-data See the full presentation of XKeyscore here:
www.theguardian.com/world/interactive/2013/jul/31/nsa-xkeyscore-program-full-presentation