You mean a complete collapse of social media, not the whole internet. The internet is a telecom ecosystem and has a lot more to it than just forums and link aggregators.
I honestly believe it might not even be such a bad thing. People were arguably better without social networks and media, and it's perhaps better to let the cancerous thing just die and keep the internet just as a utility powering boring things like banking and academia.
What would you say are the major applications of the internet? It's used for business and academia in ways that aren't going away, yes. M2M communication will stay. Social media is the largest user-facing segment and it might not. I don't have a sense of how big these sectors are relative to each other. If the largest sectors of the internet disappear, the internet shrinks a lot.