We made a change today in the way foursquare connects to Facebook. If you were previously having trouble connecting to Facebook or posting your check-ins to your Facebook wall, the issue should be resolved. You'll need to unlink your accounts and relink them to ensure that you have the proper connection.

Follow instructions here:

http://support.foursquare.com/entries/195909-why-aren-t-my-check-in...

You should also make sure that you have not revoked access to foursquare from your Facebook settings page:

http://www.facebook.com/#!/editapps.php

Foursquare should be listed and when you click "Edit Settings" next to foursquare and click on Additional Permissions, all three checkboxes should be checked.