So you built an app that's only viable 17 days ?
hey. for us it's just an experiment: are people willing to submit a single goal and show it to others? of course after 21st there are other possibilities to catch people's attention (think about New Year's resolutions).
The problem is the world will most likely not end and you will be forced to change the approach.
The other thing is discoverability If people really want something they'll do it eventually especially if they believe the world ends,with or without an app such as yours.
As with all opinions mine may be biased especially since I'm not a big social networking enthusiast.
I hope you guys get the best out of this experiment.