Facebook is no longer content with simply linking people together, it now wants to link together things - essentially by weaving the social net over the top of the existing web. Brilliant!
Why should you care?
The Open Graph protocol enables you to integrate web pages into the social graph - Facebook users can then establish connections to these pages just like they do with Facebook Pages. Your Open Graph enabled web site pages will also appear in Facebook user profiles, within Facebook search results, and in News Feeds. There is much to be gained for comparatively little effort on your part.
Open Graph is a combination of publisher plugins, semantic markup and developer API that leverages the Facebook user base (500 million and counting) in new and innovative ways.
Open Graph is currently designed for web pages that represent real-world objects such as companies, products, groups, teams, people, and places, but also includes blogs, websites and articles. That certainly covers a lot of territory, and probably includes your own set of web site properties as well.