It would be very helpful for my campaign reporting if I could set a hidden field with pre-populated data (e.g., Original Registration Source) so that I can see which signup form a user came from when exporting data.
I like this suggestion! I've passed it on to our developers for consideration.
In the meanwhile, you could add a specific group to each form, which isn't used for any other purposes. For example: create a group called Facebook and then add this to your Facebook signup form. This group could be hidden on the form and the members could still be added to your other audience groups, but this way, you'd also be able to export form-specific groups.
Another option is to create dynamic segments based on the specific signup form:
With either option, you can view/export your audience based on the form they used to signup.