IMHO, the FB app fever among users has considerably died down and also, FB as a platform itself has over time limited an app's ability to become viral. So it's

IMHO, the FB app fever among users has considerably died down and also, FB as a platform itself has over time limited an app's ability to become viral. So it's not necessarily the idea. It's the timing of the launch. For all that, a year or 2 back your app could have been a great success. So don't bother too much about your ability to evaluate an idea. It's all about riding on the next big trend.