Charles Arthur, in an interesting piece on WhatsApp groups essentially radicalising British MPs:

The dynamics of closed groups have been understood for years. The clearest finding is that they tend towards the most extreme position of their participants, something known as “the law of group polarisation”, described in a seminal paper in 1999 by Cass Sunstein, then at the University of Chicago. As Sunstein observed, it “helps to explain extremism, ‘radicalisation’, cultural shifts, and the behaviour of political parties and religious organisations”. And, he added, “it is closely connected to current concerns about the consequences of the internet”– which was not then nearly as pervasive as it is now.

This is an important piece of existing knowledge - with both the consumers and platforms starting to supplant feed-based environments with closed and chat environments, we could be swapping one set of polarising problems with another.