Politics is polarising and always has been. At its most existential, politics deals with questions of war and peace, life and death, hinge on the power political actors can muster. The notion of consensual politics or the politics of compromise is a product of liberal theories of the democratic polity, and we should bear this in mind when we query whether or not social media is polarising or not. One has to remember that citizens in the UK, for example, purchased their newspapers on the basis of that newspaper's editorial line on various questions of political dispute for at least the last 200-years. No one seems to suggest that this feature of the newspaper consumer market led to polarisation; however, the ownership structure of the media has long been a source of great controversy. For example, in the UK the Labour movement (and party) had to produce its own newspapers to get its message into the public domain and frequently extorted its members and voters to not heed the influence of the "newspaper barons."
The question then might need to rephrased as to what extent does Facebook's advertising model lead to this echo-chamber effect whereby news articles appear algorithmically? Does profit driven and advertising driven news media (from newspapers to TV to social media) drive polarisation? Would we have a more consensual politics if the news media was not governed by market forces?
Polarisation in, and of itself, is not necessarily a bad thing and certainly a feature of competitive political systems or in polities characterised by gaping class or ethnic chasms.