What's common sense to one person may not be common sense to another, and I think that's what social capital is. Social capital, to me, is the exclusive knowledge, resources, and network you grow and build from simply being connected socially to the right people.
Sometimes I think of social capital as experience. With each new person met, you can gain the aggregate of the person's knowledge. Your life's experiences multiply by association. Storytelling enables you to experience someone else's life, to learn from their mistakes and successes in short time.
There's no measuring social capital, because one person might possess a different sort of social capital that proffers different subject matter expertise than another person.
You'd think that with the advent of the internet, everyone would have fairly equal access to social capital, but a lot of people's intimate knowledge about various topics and life can't be found on the internet. They're informal observations, inconclusive but noticeable trends and patterns, and vague ideas. Furthermore, the internet might even make the gap between those with social capital and those without even larger. It is the ones with the most social capital that realize the importance of active and dynamic exchanges. Context-diverse interaction reigns as the most important tool for building social capital, I think.