But a subscription is a simpler building block, that allows for a wider variety of relationships. With Facebook, every relation must be symmetric; with Twitter, relations are symmetric by choice; you can have friends (you add them, and they add you), but also fans, without having to extend the feature set or invent new terms. Facebook, on the other hand, had to create Fan pages, because they didn't have a preexisting verb that users could overload to do the same thing.