Tara Hunt has an excellent post on Social Capital over at Horse.Pig.Cow.

the key to attracting and keeping members of a community is to offer them heaps of ways to accrue Social Capital within it. That is, to offer them more connections, more influence, more of a chance to grow their reputation, more bridging capital, more bonding capital, more access and potential access to ideas, talent and resources, more ways to display their accomplishments, more ways to do nice things for others (to build up those favors) and more access to those with loads of Social Capital of their own.

If social capital is the benefit of participation, we can compare it to the cost of participation and map the formula for community success.

Social Capital Created – Cost to participate = Community Success