Can I use Siri to update Twitter / Facebook?
Apple launched Siri beta with iOS 5 along with native Twitter integration. One thing that Siri can't do, however is update Twitter or Facebook directly. There is no built-in support for Siri to send updates to these social networking sites.
Luckily there is a simple workaround. Because Twitter can be updated via text message, Siri is capable of sending an update via text. See these instructions to configure your Facebook, Twitter or Google+ account to accept text message updates.
Once you have the right codes for your Twitter or other social networking accounts saved in the Contacts app, you can tell Siri to send a text to the account you wish to update. Apple is likely working on an update to Siri that will add more social networking features in the future.