this is a tweak that injects into the Facebook app and modifies its http traffic to prevent it from crashing. the cause is that facebook changed their REST api sometime last year, and the server now responds with status code 404 on calls to deprecated methods such as FQL, the facebook query language. the facebook app, strangely enough, isn't able to handle this and crashes as a result. a year ago, it would simply give a 400 and a malformed syntax error in the body. what this tweak does is put that error back in the body and builds a new response with the status code set to 400. the app is designed to take this, and it just silently fails instead of crashing.