Figured out what the problem was.

The problem is that the Facebook WC plugin will either see products as 'simple' or 'variant' based on the presence of any variant options.

This is fine, however when it sees a product as a 'variant' it treats ALL of the attributes as variant attributes. This isn't always true since you can have attributes that are just attributes, and not used as variants.

The reason this causes problems is that the plugin then tries to tell FB that this attribute which is not a variant, is a variant, so it's passing in a NULL array when it shouldn't be.

The file in question is facebook-for-woocommerce/facebook-commerce.php on line 866. Changing this to:

'options' => $option_values == null ? array() : $option_values;

Will send it a blank array if it doesn't have values, instead of NULL.

This causes the products to be synced properly.

I was hoping to not have to hack at this plugin, but this is what happens when they don't write things properly. Hopefully this helps someone else trying to solve this issue.