Incentivize sharing across your target platforms. A great case study for this type of engagement is Urban Hunt. Sponsored by FIJI Water, participants in the day long contest are required to open their social profiles (making your wall public on Facebook, etc...) and post bits of information including YouTube videos, status updates, check-ins, Tweets and more to earn points in the event.
Source - http://admaven.blogspot.com/2010/06/facebook-fan-value-definitive-roi.html
