Initially when “like” was introduced by Facebook, you could like only status messages and Wall posts. Then Facebook made it possible to “like” pages by replacing the “Become a Fan”. In F8 conference, Facebook introduced Like for web pages. This feature has become so popular that you can now like comments.
“Like” for comments was one of the most user demanded features. In absence of this, many people commented saying “I like [his/her] comment”.
As on wall posts, when user likes your comment, you will be receiving notifications. Similarly, others will also be able to see who has liked the comments based on the privacy settings.
“Like” was initially imported from Friendfeed and has become enormously popular on Facebook. Many users are also demanding the “Unlike” buttons. There are several groups and pages on Facebook demanding the “unlike” button which millions of users have joined. Will Facebook add the “Dislike” button too?
You can find more about like for comments on Facebook blog.