Relationship Symmetry in Social Networks: Why Facebook will go Fully Asymmetric - Bokardo
n 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. Relationship Symmetry in the Facebook model 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.
Source: bokardo.com