Before you start getting heavily involved in any type of social media marketing, it is important to understand all the different types of social media that are available and the pros and cons of each.
Services that allow you to connect with other people of similar interests and background. Usually they consist of a profile, various ways to interact with other users, ability to setup groups, etc. The most popular are Facebook and LinkedIn.
Services that allow you to save, organize and manage links to various websites and resources around the internet. Most allow you to "tag" your links to make them easy to search and share. The most popular are Delicious and StumbleUpon.
Services that allow people to post various news items or links to outside articles and then allows it's users to "vote" on the items. The voting is the core social aspect as the items that get the most votes are displayed the most prominently. The community decides which news items get seen by more people. The most popular are Digg and Reddit.
Services that allow you to upload and share various media such as pictures and video. Most services have additional social features such as profiles, commenting, etc. The most popular are YouTube and Flickr.
Services that focus on short updates that are pushed out to anyone subscribed to receive the updates. The most popular is Twitter.
Blog Comments and Forums
Online forums allow members to hold conversations by posting messages. Blog comments are similar except they are attached to blogs and usually the discussion centers around the topic of the blog post. There are MANY popular blogs and forums.
Keep in mind that, while these are the 6 different types of social media, there can be overlap among the various services. For instance, Facebook has microblogging features with their "status update". Also, Flickr and YouTube have comment systems similar to that of blogs.