It's not what you know, it's who you know. But this age-old truism about networking and getting ahead apparently needs an update, according to an intriguing post over at The Harvard Business Review's blog. It is who you know, as long as they don't know the same people you know.

An explanation. University of Virginia management professor Rob Cross, who studies the social networks leaders and organizations build, writes that the secret to your network has never been how big it is. Rather, contrary to what you might think, people who perform better tend to have more connections to people who are not very well connected. As Cross writes, "people with ties to the less-connected are more likely to hear about ideas that haven't gotten exposure elsewhere." The more your network connects you into lots of different groups of people, in other words, the more of a chance you have of being introduced to new ideas, new people and new prospects.

It's a particularly compelling concept in an age that glorifies the social network, and as so many people seem motivated by the size of their LinkedIn network or the number of Facebook friends that they have. Social media tools are useful, but only if they help us connect with people who we wouldn't otherwise interact with regularly. As many companies and organizations start deploying social networking tools in the workplace, this will matter more and more. "If we are circulating too much with people we have known forever or people who themselves are all spending time in the same meetings and interactions," Cross writes, "then we are not getting the performance impact we can from social-media tools."

In other words, the echo chamber that can make even a big network have a small effect is the same in the virtual world as it is in the real one. Leaders who spend a lot of time trying to develop their networks--or who encourage their employees to do the same--would be well advised to spend more time acquiring contacts who don't know the same people they do.