Facebook has recently made a big push to extend the Facebook experience to every webpage. Here are 2 articles that summarize the changes and its impact.
See an overview of the changes:
Zuckerberg: “We Are Building A Web Where The Default Is Social”
And summary list of the new features and impacts:
http://www.web-strategist.com/blog/2010/04/21/facebooks-crusade-of-colonization/
Facebook has over 400 million active users (and it appears to be approaching 500 million rapidly), and the service is growing at a faster rate than ever before.