Like many others, I've gotten addicted to Facebook.
I was recently invited to join a group in order to help them win a contest run by a radio station, 102.1 FM. The point appears to be that the group with the highest numbers of members ("friends") listed will receive $7000 to be applied towards its cause.
Seems simple enough, and if you already have a Facebook profile, you might just be able to help support a cause you believe in. Check out the various groups in the standings; they cover a number of different types of charities whether it's helping kids, battling cancer, animal shelters, etc. Pick one, join it, and your work is done.
The contest ends May 25th, I believe.