Hi!
Yours is the second lightbox-esque plugin in a row I've tried that has this bug (and your plugin is beautiful and works perfectly otherwise), so I'm guessing it's an issue particular to Facebook that you might be interested in...
If you go to
www.soda.40303.org/index.php?option=com_content&view=article&id=2&Itemid=6 and click on most any of the links, you'll get a lightbox with a Facebook group in it. ALL of these lightboxes inexplicably have a 50% opacity black overlay atop them, while other sites (For an example, click the link for The Soda on that page) do not.
I noticed this issue about a month or two ago; lightboxing Facebook worked fine until then (Thinking it was an issue particular to my template or that lightbox plugin, I tried both links not on Facebook and a second plugin -- yours, which I actually prefer now).
Worse case scenario I just tell these to open in a parent browser, but it'd be nice to have them as lightboxes. Thus, if there's no way around it because of some bizarre new Facebook security or privacy feature, please tell me that and I'll change how I'm implementing that section.
Thanks! Again, your plugin rocks.
-ж.