Facebook has added a Subscribe link to comments. These include comments viewed on the social network as well as comments made in the Comments Box plugin that Facebook offers on third-party sites.
The new link gives you the ability to subscribe to commenters in one click. In turn, it gives commenters another way to grow their number of Subscribers.
The link will only appear on comments made by Facebook users who have enabled Subscribers on their account. In other words, if you haven't allowed people to subscribe to your public Facebook updates, the link won't appear beside your comments.
In the screenshot above, you can see that the person's subscriber count is also visible. This doesn't seem to always happen, and is probably only shown if the user has a certain number of subscribers.
This is a brilliant move on Facebook's part, especially when it comes to the Comments Box. If you see an interesting or informative comment, you can now quickly subscribe to that person's public updates, assuming they have turned the option on.