How do you tell someone that their idea/site is bad?

Before I start, I pride myself on providing constructive criticism, but I don't want to come off an an asshole. That said, if someone asks for feedback on a website that they have already dropped a few thousand into and the whole concept is silly - what do you say???

I'm not talking about questionable ideas where you dislike the idea but would not be surprised to be proven wrong. I am talking about ideas that just aren't going to take off (e.g. something pointless like a site where you can pay a guy to throw a rock in a pond)