Facebook has begun integrating their new Subscribe functionality into Facebook Comments. It includes a small subscribe link next to each commenter's name / icon that allows in-line subscription. That does a few things: 1. It keep activity on the external site... so publishers love it. 2. It gives yet another incentive for users to comment... so readers love it. 3. And it provides more context for readers as Facebook helps sort content based on relationship, activity and 'following'.
And....
4. It is a great way for Facebook to promote the Subscribe feature which frankly is lost of Facebook.com.... findability, context and usefulness is better off-Facebook.com (just ask Twitter).
