One way of thinking about social networks is through "social objects" - the cultural touchstones and shared ideas that we use to bridge our Dunbar-constrained networks to broader communities. We all do this - use some shared object as a point of conversational reference. Here in the US it tends to be sports teams activities; in the UK there's always the weather to fall back on. (This fails in California because it's too predictable - when I came here in 1998 it took me a while to realise that saying "beautiful day again" was like saying "I see gravity's still working" to Californians).
Talking about social objects is nothing new - Jyri discussed them at length back in 2005, and danah dissected Friendster's suicide in 2006 when they killed off the Fakesters (user profiles that represented social objects like "Jack Daniels" and "Burning Man").
Where I find the 'Dunbar number' idea falls down is that social network connections, like so many other human-made things, are power-law distributed. The small number of highly-connected entities that fulfil the role of social objects are sometimes people. If you think about celebrities, they clearly fit- being able to discuss Brad and Jen and Angelina's latest shenanigans binds you in, and shows like American Idol are designed to draw on this need, giving the Faustian bargain of fame in exchange for objectification.
Different social network services can be distinguished through the different kinds of object that lead to their success. Friendster's expulsion of Fakesters, and later attempts to use TV characters is one example; MySpace's embrace of independent bands and FaceBook's initial use of Universities as touchstones help explain their divergence. LinkedIn has Companies as their touchstone, Orkut has it's communities, which are often used as badges for the users to express their identity. Suicide Girls is a blog network focused around models-as-objects, last.fm uses songs and bands, Flickr uses photos, Dopplr uses places.
Looked at in this way, James Hong's attempt to change the core objects of HotOrNot from pictures of strangers to pictoral self-tags he calls "Stylepix" seems an interesting experiment.
Making sense of the different object graphs and how they interact with the social graphs in these overlapping sites will keep lots more researchers busy, I'm sure.