Groupthink is a very well-documented source of bias in cognitive psychology. It refers to the tendency of humans to tend to agree with each other, and hold back objections or dissent even when the group is wrong.
There seems to be a balance of tensions between evaporative cooling of beliefs leading to groupthink, and extremely biased color politics.
An echo chamber is a group of media sources that parrot each other by constantly and unquestioningly reporting a single, biased point of view, thus creating the illusion of consensus.
In computer-mediated deliberation, the ideology of so-called "virtual community" has been implicated as a damaging source of groupthink. In the real world, a "community" is an ethical and political compromise of values due to a shared need for protection from bodily harm or harm to one's surrounding environment. A "virtual community" has no such natural values but still retains the mind-killing illusion of protection, and is thus liable to turn into a cult.