Social density is the number of interactions within an area.
Offline, a lot of people talking in a small room produces many fascinating behavioural changes. Online, it has a unique importance.
If your social density is too high (i.e. you concentrate too many interactions within too few areas of your community platform) it feels like a frenzy of activity. It becomes difficult to keep up and follow conversations. The community becomes a frustrating experience. It's hard to form genuine relationships. Members drift away.
If your social density is too low (i.e. you spread the same level of interactions over too many places to host them, such as forum categories, chat rooms, status updates, twitter etc...) the community feels empty. It takes longer to get a response to your posts. Members get bored and leave.
There are two challenges here. The first is to reach an optimal level of social density, the second is to maintain a consistent level of social density as your community grows. This means you need to identify niche topics within the community and create separate places for them.