This wouldn't apply to Twitter or Facebook because that is a foreign-based platform that distributes messages to more or less anyone willing to read them from any location in the world. The data is not, conceptually at least, making a return 'internal' journey to a specific recipient.
The government position as argued by Farr is that this doesn't constitute mass unwarranted surveillance and that security staff are aware of the legal limits on their powers.
Whether the security services are exploiting an ambiguity or not, the mechanisms through which the security services gain access to internet traffic remain controversial. In 2013, Privacy International lodged a complaint with the OECD alleging that telecoms firms were secretly collaborating with GCHQ in tapping their fibre optic cables.
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