What motivates people to contribute content to user-driven web sites? A blog post on the Psychology Behind Social Media does a nice job summing up the major factors? I was particularly interested that it noted the desire for group identity:

One of the main reasons that people decide to connect with others on social media channels is to have a sense of belonging to a community. They want to feel that they are an important part of a valuable group. When you focus your social media marketing campaign on your target audience, your approach should be simple. You need to include them in your communications, invite them to be a part of your relevant and appropriate online communities, ask them to share their knowledge and opinions and engage them in discussions as often as possible.

One of the primary themes of my dissertation was the power of group identity as a motivation for participation. Reading the social psychological literature on group identity, it became clear that people join groups that reinforce positive ascriptive identity — those identities that are positively valued in our society. The practical implication is that you need to organize your community’s membership around identities people feel good about: I’m far more likely to identify myself as a member of “parents of gifted children” than I am to join up with “compulsive nail-biters”.

The activities described above — inviting people to be part of a community, engaging them in discussion — are ways of helping people reinforce the positive identity you are offering. But any activity that reinforces their sense of “groupness” can be constructive, whether it’s offering a badge for their web site or adding their names to a Twitter list.