No mention here if this was the mobile app, but the IOS version will try and upload your contacts when you install messenger. If you or one of your contacts up

No mention here if this was the mobile app, but the IOS version will try and upload your contacts when you install messenger. If you or one of your contacts uploads their address book then your friendship/connection can be inferred from the emails now in their database. I'd guess your client has messenger installed if you didn't install it.