But is this a unique trait among millennials, or just the nature of social media in general?
My own experiences with social media lead me toward the latter, as I've seen Gen X-ers and Boomers similarly wall themselves off from criticism and carve out spaces where they will rarely if ever be challenged on their views. The main difference being they also get to use the age defense, wherein they can deflect any disagreement with "I'm xyz years old so you can't tell me shit."
This I agree with, but again I think that's a result of age; young people always think they have the world all figured out. Speaking as a 25 year old guy, it took a lot for me to finally just accept that I don't know shit. lol