I don't think the effect necessarily requires the agents engaged by it to directly consume social media, as the traditional media producers act as surrogates, and have their increasingly extreme views and publications validated by the echo chamber. It's inarguable that SM has fundamentally altered the news cycle. Additionally, even if only a minority of a group engage in SM, it'll likewise amplify their views which then spill into meatspace, and then propagate through more traditional lines. It turns seeds into saplings, providing fertile ground for the burgeoning crop of political insanity.

Add to this reactionary forces, whereby groups step up their rhetoric in defence against a perceived threat to their worldview, and you see that part of what spurred the non SM older brexit group was opposition to theincreasingly shrill and derogatory warnings from the remainers.

Essentially, I think social media catalyses division, regardless of whether you directly consume it or not.