1.) I didn't say Wikipedia has 5 billion uniques. I said it gets 5 billion visitors. In ad sales a visitor is different than a unique. I changed it to "visits" so avoid the confusion.
2.) Tax deduction makes the perception of something's cost free.
3.) If you can still get a product without paying for something (i.e. tipping) the price is either what you ask for ($5) or nothing. That's the definition of price uncertainty.
4.) I pointed out the problem in using NPR as a model. Federal grants and family endowments are not the best model for a blog...
5.) A donation accepting site can suggest scarcity, but the questions is how compelling will it be? PBS and NPR offer content in a very limited channel space. Most blogs and podcasts do not.
Best,
Andrew