In his book Why We Buy - The Science of Shopping, Paco Underhill tells of a large hotel in an American city that was receiving numerous complaints about their slow lifts. New motors were prohibitively expensive, so some lateral thinking led to mirrors being installed in the lifts. The complaints stopped.
I think this approach is pertinent to those of us who seek to design a better user experience online. I’m not suggesting that web users are raving narcissists, but a human-centred design approach demands that we consider these very human attributes: ego and status. These base needs (from the Maslow hierarchy) regularly demand satisfaction. As designers and creators of user experiences, we can satisfy them by reminding users of their valued input, achievements and relationships with quantifiable metrics.
One example of this being done particularly well is a new feature of the popular blogging tool, Tumblr that makes it easy for users to monitor their ‘status metrics’. In addition to keeping a running tally of a user’s blog posts and followers, Tumblr tracks the number of times another user has 'liked' or re-published a particular blog entry.

It’s all packaged into a special panel and tallied up as the user's 'Tumblarity'. Clever stuff. There's a whole host of numbers here that remind users of their input, and all of them encourage users to create more content.
This is something that leading social networks also do well. Facebook quantifies 'friends' as Twitter does 'followers'. Measuring, displaying and communicating contribution and reward is an integral part of supporting an online community. Stanford professor and author Lawrence Lessig articulated this in a recent interview: "Most companies look at what consumers create, co-create, and share with the world as some kind of free resource to be exploited in what ever way they can, but the winners in the future will be the companies that can create ecosystems in which all the participants are valued, rewarded, promoted, and empowered."
Smart and creative applications of these measures encourage users to participate more frequently and with a higher level of engagement. From here, the flow-on effect might be that a deeper relationship is formed with users (or customers) that has the potential to elevate site visitors to brand or product ambassadors. That should be a common goal for pretty much every commercial website.