Approaching 1 billion members, Facebook has a lot of interesting people, but not all of them can be friends. As with Twitter, Facebook allows users to follow posts from users who are not actual “friends” by subscribing to feeds of their public status updates. All those followed-but-not-friended people appear in a single list called “Subscriptions.”
Google+ has a whole different perspective—much more like Twitter’s. A user can follow anyone and file them in any number of Circles, whether or not that person reciprocates by following back. So someone who is not a friend of Lady Gaga can still add her to multiple circles, such as Celebs and Musicians. She could even be put in a list called Family, for fans who feel a special bond.
Google+ has a smarter approach. Ashton Kutcher and Guy Kawasaki may both be worth following, but their posts may not go together very well. Filing them in different circles allows for smarter following.