Common knowledge in social networks

Something not particularly profound, but still somehow interesting (to me, at least): I'm not a member of FacePage or MyBook or whatever, but an important part of linking with someone is sharing with them whom you're linked with besides them. From that point on, they also know whom you become linked with -- or unlinked from -- and when. Depending on the situation, that sort of information might be very interesting in and of itself.

Nothing wrong with that, given that it's a two-way street and everyone understands and accepts the rules from the outset, but it might conceivably influence one's actions. You know my connections and the changes to them, but I know yours, and I know you know mine, and you know I know yours, and so forth. What I'm fumbling at here is that this looks like a case of common knowledge, knowledge which is significant not only for its own sake, but because everyone knows everyone knows it.