I suppose YouTube is somewhat there. But it's blended with so much crap.
I think you took the wrong lesson here (seems appropriate given the subject).
Edutainment (Word Rescue and so on) wasn't unpopular because people are stupid. It was unpopular because it wasn't very good. They chased two rabbits and lost them both (education, entertainment).
Mario was more fun, better art, and probably better for your mind.
However, in the slightly earlier life of the internet BBSes and forums were all the rage, and those were actually useful, because the focus was on discussing common interests instead of everyone just showing themselves off (and forums are still around! Discourse is a modern open source implementation).
Which is probably also why people like HN, because it still follows the forum pattern rather than the social network pattern.
Some subreddits are also useful for this, when you get into somewhat less controversial corners like learning math or music theory.
Stack Overflow and its various spinoffs is probably another good real-world example.
Of course users in a hypothetical social education network would want views from their material too, but it would require the removal of sinking to the lowest common denominator from the equation. As you pointed out, that seems to already exist in specific forums/sites/etc.
Really, I guess it comes down to the users. People who want to learn will seek out education and discourse wherever they can. And those who don't seek it out, either don't want to or are unaware that they can. Is it possible to make education and sharing of ideas more attractive via a social network to that second group?
I wonder if something run by the Wikimedia Foundation or an equivalent could work. Like a MyWiki.
Gambling industries all over the world have been doing this longer than Facebook, and nobody seemed to bat too much of an eyelid.