Facebook is not good to keep anything secret, and your personal data is at stake with Facebook. There were reasons to say that always, and one more reason has now come out as the contacts list in Facebook has the phone numbers openly available for other friends in the list. Here is how you can get the phone numbers (although not everyone’s) –
In your account, go to the Account link in the right top side, which would open a dropdown list of options where you need to open “Edit Friends” for opening the Friends list and contacts.
In the Friends page, you need to click on the “Contacts” link which is the 3rd link in the left sidebar column. This would open something like a phone book, where nothing other than the name and phone number would be seen.
The phonebook has the contact number in the list of friends, and according to Facebook these numbers come from the contacts that you import using the Facebook in your mobile. A little more look into the system suggests that if you had the phone number in your profile, or else used the number to have double security for your account, that number too is listed.
This is beneficial for many, and harmful too. People who are in an emergency, and would want some of their close friends’ contact numbers, this is a good way for that. But, the same numbers exposed in this way can lead to unnecessary calls and breach in privacy.
How to Find Phone Number of Friends on Facebook