Jasper,
I greatly, greatly apologize for missing this post. I'm not sure why it got lost, but I found it when searching for an answer to another related question we were just asked.
I hope it's not too late, but I'll answer regardless. Unfortunately, Facebook's widgets only take a px parameter. We'd love it if % values worked, but that's not what they work with. With that said, you can add some CSS to your template which can make a lot of the modules work. Unfortunately, it's a little different for each module and some of the CSS can conflict with the template styles itself. We're working to get a set of good styles together that should work and have them in an upcoming release of JFBConnect.
For reference, one of the most requested modules to support responsive is the Comments module. For that, if you add the below line to your CSS, that generally makes it work how you want:
.fb-comments, .fb-comments iframe[style] {width: 100% !important;}Let us know if that helps, and again, I'm very sorry for the delay!
Thanks,
Alex