UK spy boss Charles Farr said that such platforms are classified as external communications.
The policy was revealed as part of an ongoing legal battle with campaign group Privacy International (PI).
PI said the interpretation of the law "patronises the British people".
It is the first time that the UK has commented on how the UK's legal framework allows the mass interception of communications as outlined by US whistleblower Edward Snowden in his leaks about global government surveillance.
The policy was revealed by Charles Farr, director general of the Office of Security and Counter-Terrorism.
According to Mr Farr, Facebook, Twitter, Facebook, YouTube and web searches on Google, as well as webmail services such as Hotmail and Yahoo are classified as "external communications", which means that they can be intercepted without the need for additional legal clearance."
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