More here: http://www.scottporad.com/2010/04/28/improving-dual-incentiv...
Which scenario sounds better to you:
1) Cindy Smith at George Washington Middle School wants a freebie. She sends an email to her colleagues asking them to sign up. Some do, she gets her freebie. Several colleagues who were not interested in Bingo Card Creator get emailed.
2) Cindy Smith at George Washington Middle School wants a freebie. She sends an email to her colleagues asking if any of them are current users of Bingo Card Creator. No one is. Cindy Smith gets disappointed. No one else signs up. Several colleagues who were not interested in Bingo Card Creator get emailed.
When people look for jobs, this happens often. Someone you know well works somewhere and you ask him/her to refer you. This makes it much more palatable, I believe even more than the double-sided incentive.
Of course, the downside being only people who end up on the webpage will do this. The double-sided incentive actively seeks to find more customers.