Monitor/promiscuous mode only works on WEP and open wifi networks. I assume you have a WAP network so this isn't working. As a POC you can add your phone as a network interface. To do that:
1. Plug in your iPhone/iPad into your computer via USB cable
2. Go into Xcode Organizer and find your iDevice's UID
3. Hop over to terminal and type 'rvictl -s <UID>'
4. Type 'sudo tcpdump -n -i rvi0 -s 2048 -A dst i.instagram.com'
5. Open Instagram on the device, and the packet info should appear in terminal. The rest is the same
6. When you're done you can type 'rvictl -x <UID>'