This passage in Matt Schrage’s post on the “rationalization of the attention market” does a good job explaining why social media creates blocks of people who think alike:

The rulers of the past commanded attention because they were powerful, those who receive it today, do so because they are entertaining. To consistently channel attention on the internet is to constantly be competing for it. In the individualized feed, before a piece of content is presented, it is compared against every other article, video and tweet and if, according to the proprietary algorithm that models your interests, it is insufficiently fascinating to you, you will never see it.  Even those who build up a following are still subject to these demands. Nothing uninteresting can be communicated, for the very property of being uninteresting precludes distribution on platforms that algorithmically match content to the interests of the individual. Mass media had to avoid being hated, individualized media must be loved. If the mass market required an average palatability, a perfectly legible attention market demands the extreme preference of a small niche.