Group-think is just a form of tribalism. Since the net's early days (think BB systems and IRC), it's most popular social use has been providing a space for like-minded people to exchange thoughts (which might be research, but also cyber-venting). This can be a good thing (as it was for the LGBTQ community, who could find info and support where they couldn't IRL) but also a terrible thing (think people like Vroosh).

When a tribe owns a space built around a single ideology, it can't avoid a bit of a circle-jerk. These groups usually meet to find tolerant expression of whatever niche concern, but then calcify and create their own "insider" value-systems.

Reddit is actually pretty solid overall (though many subs suffer from groupthink if the mods are all cut from one cloth).

FB is really dependent on the sort of friends you have. If you regularly mute everyone that disagrees with you, then confirmation bias will take over and you'll only ever hear from one POV. But if you have a solid enough mix of friends, family, co-workers, classmates, it can actually be a pretty diverse experience. (Also depends on what pages you follow and how much you interact with their content -- which FB will then show you more of, eventually removing the stuff that you always ignore.)

Twitter is a mixed bag. The early adopters (mostly techies and journalists) defined the culture before the masses jumped in. That's certainly given it a liberal edge. But, as with FB, your experience will really match the sort of people you follow.

There aren't really any clear advantages to groupthink once it passes a certain stage. As with the LGBTQ example, it can provide a healthy benefit when it's still in the early tolerance phase, but it takes a really self-aware and committed community to avoid slinking into a full hive. (The best cure for which is a diversity of mods and a larger base of users.)