I have a fresh Wordpress 3.5 Multisite installation. (I point out that it's fresh because fresh 3.5 installs don't use the blog.dir folder for uploads anymore, whereas upgraded <3.5 installs do). It's not online yet - it's all hosted on a dev environment on my laptop.
Anyway, as soon as I activate the Ultimate Facebook plugin the Media Library in the 'Insert Media' modal (when you're editing a page or post) is entirely empty. The media shows back up again when I deactivate the plugin.
I tried deactivating all my other plugins to ensure there was no conflict and got the same results.