So I've been “noodling” over this analogy for a while. People often complain that protocols like SAML are complex and not scaleable because they require “out-of-band” provisioning. In other words, in deployment something has to happen first, before the protocol becomes valuable. In the case of SAML, this “out-of-band” provisioning is what adds trust to the deployed network (hence the name circles-of-trust). (For all the SAML experts out there, please forgive my simplistic characterization).
One of the big selling points for OpenID is that it does not require this “out-of-band” provisioning and hence is easier to deploy and more “scaleable” (I believe the term is “internet scale”). However, one element missing from OpenID is trust. The ideal is that both OPs and RPs trust the user, but that is a lot of risk for especially the RP to take on in today's business climate.
So how does this relate to social networks? Well, in many social networks, both parties have to agree to the relationship before the relationship becomes valid. This might not be explicitly “out-of-band” but it is similar in concept. For example, if I invite someone to be a friend on Facebook, they don't show up as a friend until they have explicitly approved the relationship. Because of this factor, I am in essence building a “circle-of-trust” that is my friends on Facebook.
The “OpenID” example in social networks is AIM. I can add anyone I want to my buddy list (provided I know their AIM id) and they will be part of my network. I will also see when they are online and offline. The only way for a person to “block” this behavior is to reject presence information for all users except those on their buddy list.
My buddy list on AIM is definitely a social network of “people-I-know” but I don't think I could go so far as to call it a “circle-of-trust”:)