It seems to me that livejournal develops this model in a slightly unusual way, in that we are all gardeners of our own little patch, and we are fairly free to wander into other people's gardens. Livejournal is a patchwork of little spaces. This means the burden of reputation, and of caring for social space, is quite distributed. The work isn't ignored, it's just split into manageable chunks. And Reputation is assigned democratically - we decide which gardens we want to wander in, we decide what we want our gardens to be like.
Shirky identifies three rules of social space and four features of good practice. I think these features can be mapped on to Livejournal quite well, and explain why it is such a successful social space.
1 - social and technical issues can not be kept separate
2 - members are different from users
3 - members must have rights which trump those of casual users
In Livejournal 'members' are the journal owners. I am the sole owner of this space, while commentators are 'users'. In turn of course I am a 'user' of your space when I leave a comment in your blog. True to Shirk's model my privileges trump yours on this blog - I can delete spam for instance, and I am the only one who can post a main entry. Having this amount of ownership and responsibility encourages the whole space to be maintained, as lots of little gardens, avoiding the tragedy of the commons or the tragedy of the absent landlord (Keep Out Signs).
features of good practice:
1 - permanent 'handles' (identities or pseudo-identities), that people can invest in
2 - good works and good practice are recognised
3 - barriers to participation
4 - restrictions on scale
I think livejournal is genius because the distributed structure, and the practice of friending sets up automatic rewards for playing nice while keeping scale to manageable proportions. It mimics living social networks as closely as possible, and so I think it exploits natural primate social ability.