There has been a lot of research on social media. While there are no distinctive lines separating the different types of social media, there are some similar or overlapping styles. Two researchers, Haenlein and Kaplan, created distinct classifications in their article Business Horizons (2010). Here is their basic outline of the types of Social Media:

1. Social Networking Sites (ie. Facebook)
2. Blogs & Microblogs (ie. Twitter)
3. Social News Networking Sites (ie. Digg)
4. Content Communities (ie. YouTube)
5. Collaborative Projects (ie. Wikipedia)
6. Virtual game-worlds (ie. World of Warcraft)
7. Virtual social worlds (ie. Second Life)

When it comes down to social media types, people tend to use the type that they find familiar or comfortable. A variety of these several different types are commonly used, however, to be successful, the audience still needs to be considered. It is a waste of time if you are maintaining a Facebook page, when your entire audience happens to be on Twitter. This is why when considering your audience, and what social media they are using, then you will have a better chance of connecting with them.

Sources:
Kaplan Andreas M., Haenlein Michael (2010). “Users of the world, unite! The challenges and opportunities of social media”. Business Horizons 53 (1). p. 61