Just my personal opinion: users don't care if your app is open sourced or not (Unless there's a clear reason for it to be open sourced, for example providing transparency in cases user privacy is critical)
Like I said it only makes sense when you feel the need to make it open source. Some of the reasons I could think of:
1. Your app has features that are very privacy sensitive, so you want to let your users know there's nothing fishy going on. 2. You're not building the app because you want users, but as a learning experience. Hey maybe you can even use your public code as your resume (Then again, this can backfire if your code is not exquisite) 3. Marketing stunt (This won't work in most cases, so if you're thinking of this don't do it) 4. Your app has server aspect and you can't afford to run it anymore so thinking of shutting it down, but you want to do your users a favor.
Community involvement and participation will NOT come just because you open sourced it. In fact you should assume it won't happen. This is true for even extremely successful open source projects. It's only less than 1% of people who contribute and 99% of freeloaders.