I have followed up on the Andrew Marr Show story from Buzzfeed about pro Labour stories on UK Facebook. Main story HERE links to another on "Alt-Left sites" and one on Corbyn not helping some print journalists with access to events.
Suggest you follow Buzzfeed for more on this. My guess is that social media are still not strong enough to counteract the Fleet Street influence. But there could be a dent over the next month that showed up on a larger scale l;ater.
Meanwhile I do think confidence in newspapers and TV may have challenges. In Exeter we had a wrap around advert for the Conservatives in our local paper on the morning of the local elections. All quite legal apparently but I worry for what happens to local news as fewer people buy such publications.