It came along when we were all high school/college or so, and this big platform to refind old contacts was a surprising new experience.

Then all of collectively went into the workforce and suddenly had public images to manage and so the whole thing very quickly became yet another advertising channel via tragedy of the commons.

Reconnecting with old acquaintances is just one (outdated, as you mention) use case of social media. They have become much more subversive (and profitable) as media/advertising engines.

They'll survive as long as companies are willing to put money into advertising and enough people are influenced by (not even "suckered by") the ads. So basically forever.