I think #3 is closest to the mark, but I don’t think it’s caged to young adults.
For example, if you were to look anywhere on Reddit and found yourself in a thread that just barely, tangentially, almost-not-in-this-plane-of-reality touches on something related to law, a hundred people will show up to give you all sorts of the most inane and dangerous legal advice.
Granted, for something like legal advice you: 1) shouldn’t go Reddit; and 2) should search for an attorney. That said, there are (were?) some places on Reddit where you could find advice or discussion attached to the reality shared by the rest of us. But that isn’t the current draw of the site to the masses.
I’m one of the people who (until this past week) used Reddit in a technical capacity.
That shouldn’t be taken as “I get my solutions from Reddit.” Rather, I posted and consumed niche technical information for unusual problems. There were (are?) a boatload of smaller, vendor specific, etc subreddits that _did_ (do?) have smart people who collaborate or rubber ducky tricky issues.
Most of Reddit is not and was not that.
And as I type that, I realize I must apologize for sort of hijacking your reply with a response to the parent comment. I’ll leave this and have prepended a direct response to the points you raise and added a reasonable segue.