I mean, think about this for just a sec: LinkedIn, Twitter, SnapChat, Instagram, the list goes on and on. The VERY first thing these platforms will do after you've installed the app and logged in is to ask you to share all of the information in your address book. Ever wonder how these social networks got so big so fast? It's remarkable how much of their growth is based on convincing everyone it's totally fine and normal to give away all of the contact information given to them by friends, family and acquaintances.
@jurjen_heeck With modern phones, you can just deny them the permission to access the data.