In my opinion the cyber threat to the issues raised in your post isn't primarily a military threat but rather a political and social one. In both the US and the UK there currently exists a feeling of division both within the nations and in respect of long standing relationships with countries that have been considered allies throughout my life. This ultimately weakens both nations and reduces their ability to influence the rest of the world, thus handing strategic advantage to the likes of China and Russia. Given that the political division we're seeing is advantageous to these nations, I see no reason not to believe that both are using social media and other methods to create and perpetuate this state of affairs.
Unfortunately the current political leaders in both nations are in their positions because of this division, so they have no interest in calling it out or seriously investigating the sources of it.