(I have posted this at the ACF forums too: http://support.plugins.elliotcondon.com/discussion/797/incompatibility-with-facebook-comments-plugin)
The Facebook Comments plugin is incompatible with the Advanced Custom Fields plugin. When Facebook Comments is enabled, Advanced Custom fields doesn't save all the entered fields. In particular, I'm looking at Repeater fields: I add multiple fields, then upon saving they disappear.
Enabling PHP error messages shows this:
Warning: Cannot modify header information - headers already sent by (output started at /home/lvpmedia/clients.vanpattenmedia.com/susanegan.net/wp-content/plugins/facebook-comments-for-wordpress/facebook-comments.php:142) in /home/lvpmedia/clients.vanpattenmedia.com/susanegan.net/wp-includes/pluggable.php on line 934
Disabling Facebook Comments solves the problem. Hopefully it's a quick fix to make the two plugins play nice!