How the communities like facebook,orkut can able to track down our mail contact list

How few social communities can 开发者_如何学Pythonable to track down the contact lists from our mail id alone.Is there any MOU or its open for all.

Can anyone explain this.

Thanks Lokesh.

This question deals with Facebook and GMX specifically: Importing Facebook friends with API

They shouldn't be able to track down contact information with the E-Mail address alone, though. All the services I know ask you for your user name and password to log in to the actual service.

If you're seeing contact suggestions without giving the network your E-Mail credentials, what probably happens is that they cross-reference your address against other people's contact lists.

If i am not wrong they also require password for authenticate and after they accessing their CSV contact file or some other format that has been provided by the Application