  We introduce and study a general model of social network formation and
evolution based on the concept of preferential link formation between similar
nodes and increased similarity between connected nodes. The model is studied
numerically and analytically for three definitions of similarity. In common
with real-world social networks, we find coexistence of high and low
connectivity phases and history dependence. We suggest that the positive
feedback between linking and similarity which is responsible for the model's
behaviour is also an important mechanism in real social networks.
