Symmetric- (Facebook, LinkedIn)
With these types of platforms you can only interact with people you know ad who have accepted your invitation. There isn't really a possibility of one way friendships like you get with Twitter. You can however like a page on these types of sites but the interaction is not like what you would get with your friends.
Asymmetric- (Twitter, Google+)
With these types of sites you can follow people that you do not personally know. There is the possibility of one way friendships. You are able to find somebody famous on Twitter and follow them to learn more about them. Asymmetric platforms I would say are the most popular in digital marketing because people can post content that anyone can see even if they are not actually connected to that page. A company can tweet something and anyone can see it it can be retweeted and then it gets spread very quickly.
Media- (YouTube, Flickr, MySpace)
These types of sites are more focused on the media experience like a brand creating a YouTube profile to create videos that they hope have the possibility of going viral. This is the best way to get actual examples and show people using the products instead of just a picture.
Location- (Foursquare, Yelp)
As the name states these types are made up of location based user interaction. You can check in at a place and your followers can see where you are or where you have been. Businesses give out incentives to be in select locations and specific times. A business can also tell a story through the location history of your past customers.