People don't divide their time equally between apps. I have apps I haven't touched in a year. I would venture to guess the FB app is only behind phone and SMS

People don't divide their time equally between apps. I have apps I haven't touched in a year. I would venture to guess the FB app is only behind phone and SMS in terms of usage. This means that, yes, review are far more likely to be left for the FB app and, within that user base, those with issues have far more motivation to leave reviews than those who are happy. Again, not saying the FB app is good. I barely use it. Don't know. What I am saying is that review systems in general are very broken.