Social media is a form of large scale online communication between friends, family, acquaintances, and strangers. It lets you connect with friends: updating, posting, sharing. Facebook and twitter are the biggest examples of social media – an overwhelming number of people all sharing their ideas, experiences, and values.
I do not think that blogs are a form of social media. While facebook and twitter let everyone contribute equally, a blog is simply one (or more) designated authors. The public can only comment on the blog, staying on the topics that the authors chose. If everyone had access to add to and post on the blog, I would consider this to be social media.
While a company or individual creating a “facebook page” may feel similar to a blog, I consider these facebook pages to be social media and not blogs because they are already in the social media space. People can share the facebook page, tag others, let all their friends know that they have “liked” it, all through the same website in the same space.