I’ll tell you what, if you are able to write up complete instructions for how to make this work with Facebook, I will happily send you a discount for a free upgrade to pro.
See if these instructions for getting the proper HTML work for you.
1. If you have not done so already, create a new list that is “not linked”.
2. Click on the subscribers tab and next to that list you created will be a “form” icon.
3. At the bottom of the form page for that list will be an Iframe area with the Iframe code. That should be what you need for Facebook.
Once you have the code, I’m not sure exactly what to do with it on Facebook, although it does look like there are numerous Iframe applications. If you can write of the instructions, my offer stands. You can reach me outside the forum at doug at sendpress dot com. I would love to be able to add those instructions to our knowledge base.