In essence, if both js libraries are loaded asynchronously, do you wait for jQuery’s document ready? Or Facebook’s init? If it loads in the wrong order, you’re screwed.
Well, with LAB.js, why not load everything asynchronously and force one to go first? That was my solution, which has the benefit of using a loader for all your js, speeding everything up. Just for fun, I’m adding jQueryUI to the mix. You can’t use the UI with the parent library.
Here is the code in a gist. Keep in mind, I’m more of a backend programmer so don’t be shy about commenting with improvements or corrections.