Social media sites (sometimes also called Web 2.0) involve people sharing content, links, photos, conversations, videos, etc.
There are hundreds of these sites - some of general interest, others targeted to more specific audiences. Examples include FaceBook, YouTube, Flickr, Squidoo, Digg, StumbleUpon, Del.icio.us, etc.
Social media sites are a new tool in a web publisher's marketing arsenal.
Because so many people gather on these sites, sharing information, photos, links, posts, etc. they are a prime place to tune into the conversation of your niche.
Many niches have their own targeted social media sites. But the larger, more general social sites also nearly always have a group or category for any topic imaginable.
Go to some of the primary social sites and search for your niche's main keywords/phrases. You'll most likely find at least one group of people already there - posting, chatting, commenting, voting, etc.
Search for your keywords/phrases on these sites and see what you can find: