Today's Resource: Designing for Social Traction
vospe.com
vospe.com
To sum up a few points: Use lazy registration (delayed to as late as possible), first-time user help, gradual profile filling. Remove features that encourage mindless hoarding (of points, ratings, reviews, whatever) and design for the flow, ie help the user improve/learn new things but don't make the experience too challenging.
Secondly, wow. This is a seriously worthwhile click. I'm only about halfway through it, so there's still time for it to go downhill I suppose, but while some of the tips are more commonplace (delay registration as long as you can), others were downright insightful (Instead of asking "Can people fill out our sign-up form?" Ask "Are people motivated to care?").
It's long, so save it for later if you're in a hurry, but I recommend against skipping it altogether.