Advanced segments allow you to isolate and analyze specific kinds of traffic. When you create an advanced segment, it is available across all websites in your account. For example, if you create an advanced segment to view traffic from only Facebook, you can use it to see all of your Google Analytics data based on visitors coming from Facebook. To create an advanced segment, go to any profile in your Google Analytics and click on the Advanced Segments button.
While Google Analytics does provide a social report under Traffic Sources, they leave out a lot of niche social networks and lump major networks like Facebook in with traffic from social bookmarking sites, blogging sites, and Q&A sites. With my custom segments, I cannot only view the social media networks I want, but also all of my Google Analytics data based on traffic from those networks, and I can compare it to the rest of my traffic as a whole