We've posted earlier regarding the usefulness of a company blog on your web site.  Done correctly, it can be incredibly helpful.  There are some pitfalls though. Chris Lister put together an excellent post pointing out some of the worst offenses. As his commenters have already pointed out - these suggestions are just as relevant to other social media mediums.

The post is packed with great information and well worth the read. My favorite.... Greed. Don't constantly try to monetize your readers. Provide value your readers will appreciate. "Don’t be the Shamwow guy"... Love it.

Read Chris' post.