I started this as another protip comment on this FriendFeed thread about Reader likes but it got kind of long, so here goes:
Reader recently launched liking (and a bunch of other features). One of the nice things about liking is that it's completely public*. It would therefore make sense to be pretty liberal with liking data, and in fact Reader does try to expose liking in our feeds. If you look at my shared items feed you will see a bunch of entries like:
<gr:likingUser>00298835408679692061</gr:likingUser> <gr:likingUser>11558879684172144796</gr:likingUser> <gr:likingUser>07538649935038400809</gr:likingUser> <gr:likingUser>09776139491686191852</gr:likingUser> <gr:likingUser>02408713980432217881</gr:likingUser> <gr:likingUser>05429296530037195610</gr:likingUser>
These are the users that have liked. Users are represented by their IDs, which you can use to generate Reader shared page URLs. More interestingly, you can plug these into the Social Graph API to see who these users are.
Liking information isn't just limited to Reader shared item feeds. If you use Reader's view of a feed, for example The Big Picture's, you can see the
<gr:likingUser> elements there too. This means that as a publisher you can extract this information and see which of your items Reader users find interesting.
For now liking information that is included inline in the feed is limited to 100 users, mainly for performance reasons. That number may go up (or down) as we see how this feature is used. However, if you'd like to get at all of the liker information for a specific item, you can plug in an item ID into the
/reader/api/0/likers API endpoint, and then get at it in either JSON or XML formats.
* I've seen some wondering what the difference between liking, sharing and starring is. To some degree that's up to each user, but one nice thing about liking is that it has less baggage associated with it. We learned that if we try to redefine existing behaviors (like sharing) users get upset.