It is far too soon to discuss the legacy of Facebook. Indeed, since the platform constantly changes the way in which it works, it will be some time before we have a settled notion of what a "Facebook effect" looks like. However, we can say, on the basis of its performance so far, that it tends on the one hand to intensify political engagement among followers and on the other hand to polarise by cutting people off from mainstream news. There also appears to be an additional and rather worrying effect which has been referred to as the "reinforcing spiral". This means that, in simple terms, people become so cut off from alternative views that they begin to over-estimate the strength of their own analysis. This then leads to a kind of pack mentality in which they gather together to attack those who seem to disagree with them. This probably at least partly explains the 'attack' politics of recent months in the USA and the UK.
The big change from reading partisan newspapers is that those who are using social media for news, tend to only see material that they either agree with strongly or that they are being urged by friends to disagree with strongly. The lack of context means that their views are un-challenged by moderating influences from elsewhere. This narrow news funnel is quite unlike anything that we have seen for the last half century when network television dominated news consumption it is also different from the pre-television period because people tend to receive social media news in isolation and without the moderating influence of friends and co-workers.