How many "heavy" facebook users are there really? I mean imo social media is only worth it to people who have social lives. The girl who NEVER goes out isn't going to be too concerned with what her other girl and guy friends are up to on facebook.
The girl who does occasionally go out will post pictures of herself and her friends on facebook, browse through her home feed and stalk pics of her other friends.
If facebook dies, the heavy users would still be social people. They would just use instagram or maybe google will swoop in a develop some sort of social life sharing app. And if all social media dies, then those social people will prob just have tons of online and real life photo albums hanging around. Don't see it being an impediment to having a functioning social life.