Ask HN: Major Forms of Social Gaming?

Doing a bit of personal research, mostly to scratch an intellectual itch, but also because there's a few ideas I've been working on, and I want to compare it against what's out there right now.

What does HN think the major social gaming archetypes are? We've got MMOs (both subscription-based and Korean-style freemium), compulsion-loop Farmville games... what else?