I'm old enough to remember a day when the "social media" that I used was a set of phpBB forums paid for by one or more of the members because they wanted to host the community. Nothing on the modern ad-supported web comes close to the dynamic of friendship and camaraderie of those community-supported forums—if anything the new platforms are a great place to ruin real-life friendships rather than create new ones.

So, no, I don't think the web needs to be made "sustainable" in the sense you seem to mean. Things were better when people sacrificed a bit to keep their communities alive.