I've been thinking a lot about why people share in communities, so this was a cool take on things. I'd tack on to the 3 P's that communities need members who are:
1. Motivated.
2. Empathetic.
Maybe these are no s
* sherlocks. But I say motivated because people need incentives, whether it's the prospect of making money or building a reputation or getting recognition. There is something everyone is after, and many of even the best communities have some sort of reward system (HN & Quora off the top). And empathetic because members need to be able to relate to each other. If the sharer has high empathy then maybe less so. Here's a longer winded version of what I wrote:
http://marketinghead.tumblr.com/post/31338994971/why-shareAs I write this, maybe the closer community members are sympatico, the more likely you get community wo incentives needing to be so significant...