There is a known issue with forms in the Facebook mobile app, especially on iOS devices, and the target="_blank" attribute. We're not entirely sure why, but it could be due to the fact that the Facebook app browser doesn't have tabs.
When submitting the forms, a message error like this will appear:
An error has occurred, please try again.
Note that this attribute is not usually found in our HTML form code, but it may have been manually added by your web team when they embedded the form on your site.
To correct the situation, simply remove the target="_blank" from your form code.