Blogs are a big staple right now when one looks to define Social Media. Sure you have Facebook, Twitter, LinkedIn, Friendfeed, etc., but without a blog, you have no home base.
Before starting a blog, you have to decide, what will you blog about. It makes no sense to be a hose and tube fitting company and blog about air conditioners (an unlikely scenario, but you get the point).
Next, you have to make sure the person you appoint as the blog’s administrator knows what they are doing. They have to have a sense of web design, copywriting skills (blog typos happen, but an excessive amount can ruin a company’s credibility), and be web savvy.