While you can’t tell Siri to update your Facebook through the official Facebook app, it is still possible. The process is similar to setting up Siri to update Twitter.
Teach Siri To Update Facebook Status:
1. Setup and verify your mobile number through Facebook (link)
2. Add a new contact named “Facebook” in your Address Book with the number 32665
3. Start Siri (by holding down the home button for a couple seconds or putting the phone up to your ear)
4. Say “Text Facebook” and whatever you want your status to say afterwards.
5. Siri will now display your message and ask if you are ready to send it.
6. Say yes
7. You have now changed your Facebook status using Siri
