Here's how you can use Siri to post to Facebook in iOS 6 on your iPhone, iPad or iPod Touch.
1. Make sure you have your Facebook account set up in iOS 6's settings. If you don't and you try to post, Siri will remind me you and you can click the Facebook Settings button to set it up.
2. Long press the Home button.
3. When Siri activates, say "post to Facebook."
4. Siri will ask what you want to say.
5. Speak your message.
6. Siri will show you what the message looks like and ask you if you are ready to post it.
7. Say yes or tap the Post button if you want it posted.