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[Tracking User's activity out side the app] email, phone contacts..etc

One of the way Facebook recommends friends is
‘contact’ on our phone.

Anyone have any idea how to do this?

Moreover, how Facebook can track our email? ( I heard it recommends freinds based on our email activity)

I would appreciate a lot if you can help me with this concept!

Thanks.

Major bump. I’m interested as well. Did you ever find out anything?