The FBI is tasked with cases on US soil involving US persons.
I'll just quote the lead:
> The US military is developing software that will let it secretly manipulate social media sites by using fake online personas to influence internet conversations and spread pro-American propaganda.
And more recently, GCHQ's JTRIG program. [2]
Again, I'll quote:
> (The core self-identified purposes of JTRIG is to) use social sciences and other techniques to manipulate online discourse and activism to generate outcomes it considers desirable
[1]: http://www.theguardian.com/technology/2011/mar/17/us-spy-ope...
[2]: https://firstlook.org/theintercept/2014/02/24/jtrig-manipula...