In real life it is possible to judge the mood of a group and avoid or defuse contentious situations. In addition, such groups are usually transient. Real people, especially friends of friends or other passing acquaintances, don't usually follow you home, and then to work the next day, and the next, constantly telling you why your opinion is wrong.

Online, there is no escape short of opting out entirely.

In all, social media gives a megaphone to strident opinions, and has a chilling effect on everyone else.