Whenever you connect your Social Stream app to Facebook, Facebook generates a secret code for the Stream called 'AuthToken' that allows the app to retrieve your posts and images. This token has an expiration date to protect you from 'malicious' apps.
Every now and then, Facebook will invalidate these tokens so you'll have to consciously re-approve the apps you're really using.
What should I do?
1. Open Stream settings.
2. Remove Facebook from your stream (if you have multiple Facebook streams , remove at least one). Then click on Facebook again.
3. Approve Social Stream app and add the stream you have just removed.