The SEO world, particularly the blogosphere and forum chatter, is overwhelmed with talk of how to leverage social media sites for SEO value. Tonight, I'm curious - which types of social media platforms do you actually get the most direct SEO benefit from? Think classic SEO - higher rankings and more search traffic. Which slice of social media sites help you achieve this goal?

I'd love to hear your examples, too. If you said Wikis - how? What did you do on Wikipedia that resulted in more search traffic for keywords that bring you high quality traffic?