The app uploads your contacts, and probably monitors many other things that you don't want. Firefox won't let Facebook do this. And I prefer Firefox as Chrome will do similar things for Google - not the contacts, but keeping history of websites you visit etc.
But in this particular case, this is something that I want it to monitor automatically in most cases, and it provides a lot of useful information based on this monitoring.
It's an invasion of privacy for either you or (if you don't care) your contacts' privacy. You're probably on the wrong forum if you don't consider these "features" to be overreaching.
