The SDK has not been initialized, make sure to call FacebookSdk.sdkInitialize() first.

一定要进应用的时候确保首先调用facebooksdk.sdkinitialize（）

所以在application中oncreate方法中调用以下方法：

FacebookSdk.setApplicationId("你自己的id");
//初始化Facebook SDK
FacebookSdk.sdkInitialize(getApplicationContext());
if(BuildConfig.DEBUG){
    FacebookSdk.setIsDebugEnabled(true);
    FacebookSdk.addLoggingBehavior(LoggingBehavior.INCLUDE_ACCESS_TOKENS);
}