I'm trying to develop an app (profanity filter) that would allow me to scan incoming traffic/packets coming into an iPhone before it goes to an app such as Facebook, Twitter, Text Messaging, etc. Is this possible and if so, could someone please provide me with a starting point?
It's not possible on non-jailbroken phone, unfortunately.
To do so, you need access to network stack (so that you're able to pass through or discard packets). This is commonly done in kernel space of OS (pf, osx firewall, littlesnich are examples). There's no way to get into kernel space on vanilla iOS device.