Facebook announced today on their developer blog that websites can now add the subscribe button.
How It Works
The Subscribe button for websites works just like the button on Facebook; once clicked the user will begin seeing the public posts of the person they have subscribed to in his or her News Feed. The subscribe action is also shared — allowing others to subscribe directly via the News Feed stories, and further increasing viral distribution.
For info on how to add the button and see example sites, click here