https://developers.facebook.com/docs/ios/share/cn
https://github.com/fbsamples/ios-howtos
https://developers.facebook.com/docs/ios/getting-started ios6以后http://stackoverflow.com/questions/18459252/how-to-post-on-twitter-by-hiding-slcomposeviewcontroller-tweetsheettweet-instan/18460093#18460093
http://stackoverflow.com/questions/12503287/tutorial-for-slcomposeviewcontroller-sharing
http://stackoverflow.com/questions/9092142/ios-uialertview-button-to-go-to-setting-app
发现必须要isAvaliable这个方法判断一下,原因当settings中设置了facebook账户, composeViewControllerForServiceType方法返回非nil,否则返回nil.