https://developers.facebook.com/docs/ios/share/cnios
https://github.com/fbsamples/ios-howtosgit
https://developers.facebook.com/docs/ios/getting-started ios6之後http://stackoverflow.com/questions/18459252/how-to-post-on-twitter-by-hiding-slcomposeviewcontroller-tweetsheettweet-instan/18460093#18460093github
http://stackoverflow.com/questions/12503287/tutorial-for-slcomposeviewcontroller-sharingapp
http://stackoverflow.com/questions/9092142/ios-uialertview-button-to-go-to-setting-apppost
發現必需要isAvaliable這個方法判斷一下,緣由當settings中設置了facebook帳戶, composeViewControllerForServiceType方法返回非nil,不然返回nil.