As with any online project, it is good to diversify. Putting all of your eggs in the proverbial basket is just asking for trouble. We are doing very well with YPN, and it is very tempting to switch all of our sites over to it, but I know this roll with them will not last forever so we have other sites devoted to other revenue generators. I don't think YPN will die; online advertising is growing and is where the money is. YPN sees what Google is doing and wants to do the same. The revenue may drop, just as it did with AdSense, so it is always a good idea to test other means of making income.