Same thing happened to me. Here is how I've solved the case.
Using the FTP client, I went to /wp-content/plugins and renamed 'facebook' folder to 'facebook-dis'. Now, I could've accessed the WP administration. Indeed it proved that the 'facebook' plugin was deactivated by this action.
Having been there I supposed I got some conflicting plugin. There it was - 'Facebook Comments'. I deactivated that, then got out of administration, then went back to FTP client and renamed back 'facebook-dis' to 'facebook'.
Got back to administration and re-enabled 'facebook', whereas everything was working.