"It’s not real life, of course, because people overwhelmingly post about the good things whereas all the crappy, dull or deep stuff doesn’t get mentioned."
I think social media is what you make of it and who you stack your social media accounts with. If you stack them with people who only post how awesome they are, this is what you will get out of it.
My Twitter feed is a rollercoaster. I have people who share their awesome moments and their horrible moments and their thoughts and hopes and dreams and their humour. If you "follow" just anyone just because you want to be popular and have a lot of followers, you ARE going to get a lot of junk and no real content. Rather than "stopping our externalizing" I think that we, as the viewers of our social media feeds, need to take responsibility for the kind of people we choose to have in our feed.
Honestly, trying to tell people what to or not to post is like censorship. Stop burning the books and instead make a choice not to read them. Fill your life with content that makes you happy.