You could also hook into when the website does the login and read from the DOM the username and password and save it in the offline storage, and load it back into the dom next time. ( if you need help interfacing with the dom tell me, i've already done something like this)

another option is to build the POST request properly but I am not sure how easy that is to do in the qml webview.

Be advised that both options are a bit on the gray area with regards to the facebook "no screen scrapping" policy...