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There's a massive difference between topic-centered, article-oriented forums and social-oriented "life walls", in which people are drawn to share their personal life and basically have an "online life hub".

One is centered around the topic, the other is centered around the person.

[raverbashing]

Reddit is topic oriented and nonetheless is social media. Because the curation of topics is done by the users (same thing here, with some content created by the users themselves)

Pinterest is about curation of content and it is social media as well

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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.