User can either allow Facebook application to access his basic information or detailed information including his contact details. If a user is more interested in his privacy he can select the option "Access my basic information".
Why we should keep our contact details private in Facebook
Though the true spirit of Facebook is sharing and exploring new friends, some times sharing our private information with unknown person or programs can harm us. Still many users use their real information as the passwords or pass-phrases for their sensitive accounts and if they share their private information with everyone, that can harm them. Another issue is about the agencies who are collecting contact details of people and sell that data for money to third persons. Those may be marketing firms or survey agencies but why should we share our contact details with someone who is unfamiliar to us ?
1. Join Amber alerts in Facebook
2. Malicious activities in Facebook