title: How do apps like Path and Instagram tell if contacts are also using the app?
body: <p>I am interested in building an app that has a Find Friends by contacts function similar to Path's or Instagram's but am not sure how it's done.</p>
  
  <p>I know Path and Instagram upload your address book to their server, but how are they able to tell if a particular contact is also using the app? Does it also upload the user's phone number and thus match by phone numbers?</p>
  
  <p>CLARIFICATION: I do not save email addresses (or any info other than the contact's phone number) in my address book, yet somehow Path and Instagram are able to tell if that phone number belongs to a user on Path / Instagram. How is this?</p>