To be able to update your Facebook status with Siri, first navigate to Settings > Facebook on your iPhone to make Siri work with Facebook. Then add your Facebook account details, and you can use Siri to post on your timeline.
Here you’ll find settings for chat and message alerts, photo and video, and notifications.
Facebook Account Details
In the empty field you see on the left, you can fill out your Facebook account details.
Allow These Apps…
Here you’ll find the iOS apps that have been granted access to use your Facebook account.
Update All Contacts
Choose this option to provide Facebook with email addresses and phone numbers from your contacts so that photos and user names can be updated.