A counterexample is Causes on Facebook, a for-profit entity which utilizes social pressure to drive donations to nonprofits and has its own successful "tip" system.
My post was about using this model to support blogs and podcasts. Not bona fide charitable organizations. I actually have experience in doing successful online fundraising for those kinds of groups.
Agreed, I thought it would be interesting to mention the tipping strategy that the for-profit entity Causes uses at the same time they are helping out nonprofits.