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As I said, it's about the difference between "topic-oriented" and "person-oriented".

Pinterest has a very different demographic which is more likely why GGP hadn't heard of it, but I would put it in the same bucket as Reddit/HN. Facebook, Twitter etc however are person-oriented.

The only thing all these have in common is user-generated content. But at that point you can call anything with user-generated content social media. Forums? Sure. Newspapers with curated user-submitted articles? Why not.