Clipped from Bokardo.com:
In general, there are two ways to model human relationships in software. An “asymmetric” model is how Twitter currently works. You can “follow” someone else without them following you back. It’s a one-way relationship that may or may not be mutual.
Facebook, on the other hand, has always used a “symmetric” model, where each time you add someone as a friend they have to add you as a friend as well. This is a two-way relationship, and it is required to have any relationship at all. So as a Facebook user there is always a 1-1 relationship among your friends. Everyone who you have claimed as a friend has also claimed you as a friend.
Andrew Chen recently described one advantage of the Twitter model. It allows 4 types of relationships, while Facebook only allows for two. The two relationships of Facebook are “friend and Not Friend”. The four relationships of Twitter are:
People who follow you, but you don’t follow back
People who don’t follow you, but you follow them
You both follow each other (Friends!)
Neither of you follow each other
(You can follow me on Twitter, if you want, here.)