"I still don't get why businesses such as Facebook and Google can be deemed exempt from responsibility as to what content gets posted on it "
It's really simple and also explained in the article as being ALREADY part of UK legislation. If there is any sort of promotion, prioritisation or any other type of intervention on the part of the 'platform' with regard to postings and visibility thereof, then it no longer has the protections of 'just being a platform'. Once you start to pick and choose which posts to show (which FB for example clearly does) then you no longer have the excuse that you don't know about the content. Because clearly the promotion is made based on content - what is going to generate most comment / shares.
FB, Google et al can't have it both ways. They claim to their advertisers (their REAL clients) that they can manage giant volumes of data in enough detail to microtarget users, and in the same breath they claim to regulators that there's too much data for them to oversee.