Facebook is adding a subtle, but much requested feature — it’s letting users “like” comments. So if someone says something particularly witty in response to your most recent status update or photo, you and others can acknowledge it.

When a person “likes” someone else’s comment, they’ll get a notification that can be sent to their e-mail. Others will be able to see who “liked” the comment if their privacy restrictions allow it. It’s not clear whether “likes” on comments will be factored into the news feed or the order in which comments are shown. (We’ve asked Facebook about this and will update when we hear back.)

Facebook first brought “likes” on-board in early 2009 after seeing them take off on FriendFeed, another social-sharing service it eventually acquired in August of last year. “Likes” have become a core part of Facebook’s experience and its ambitious plans to organize the web. When users express preferences for brands, content or even other people through clicking the “like” button, Facebook collects even more data about people’s affinities.

Paul Buchheit, a Friendfeed co-founder who later joined Facebook through the acquisition, explained the original thinking behind the “like” button last month on-stage at the Web 2.0 Expo in San Francisco:

“We created the ‘Like’ feature in FriendFeed because I realized that people wanted an easy way to let others know that they saw what their friends posted and appreciated it. Putting in those simple little gestures is very powerful.”