I don't think it's a good idea to just quit if you have a new idea that you think is better. I say this from experience. A few months back I was convinced that we should drop our iPhone apps because we weren't passionate about them. We weren't making that good of money on them either. I was really passionate about a book sharing website (shower idea that you wrote about) and I wanted to devote my efforts to it.
Fast forward to now, had I have made that move, it would have literally cost me thousands - as the apps are really gaining traction. I blogged about the book sharing website twice and it seemed to generate little interest.
My criteria would be: a) If you aren't getting ANY traction in a predetermined period b) when you talk/write/tweet/blog/speak about your idea or product and very little seem interested, it might be time to quit or pivot. My 2 cents.
We're undergoing a massive internal change that's pretty much akin to starting an entirely new business within a business, simply because what we're doing has less potential growth than what we're looking at doing, which is new (which is rare in our industry).