the other good thing about 'viral' startups, is that you get instant feedback about whether something works; you don't have to hang around to see if people like it or will use it.

Max Levchin told me that he would never bother with a start-up that wasn't viral in some way for this same reason.

If your product is inherently viral, but isn't taking off, does that mean it's a bad idea? Or just that it hasn't tipped yet?

Probably means you're missing some critical usability feature, i.e. you're not solving the whole problem. Few ideas are all bad or all good, and things rarely tip all by themselves. But people don't use a product unless it solves their whole problem without making them hunt for missing pieces - that's really the difference between "project" and "product". If you're missing a piece, it'll never get off the ground.

The missing pieces are usually pretty subtle too - FaceBook took off largely because it was explicitly designed around existing physical social networks, like colleges. Many other people had done similar sites years earlier, but they always focused on building a social network out of thin ether, rather than leveraging offline social contacts.