I assumed they were doing it not for the donors, but for potential recipients. For example, suppose someone you know needs a kidney. You could look and see which of your friends would be willing to donate one, or ask them to post a message to their friends and see which of your friends-of-friends could donate. It's probably a lot more efficient and people are probably far more likely to donate to someone they know personally. And for the recipient it's probably a much faster and easier process than having to wait on a donor list.

Eventually I could see the same with bone marrow and blood type where it's a donation that you can make and keep living, but where millions of people die because there aren't enough donors.

P.S. Go register to be a bone marrow donor: http://marrow.org/Home.aspx