Evolutionary Page
This page will load 3rd party script files through "frame-in-frame" method which is not so new technic but was experimentally implemented into the facebook SDK as truly asynchronous loading.
Navigation Timing
Here's the timing of DOM ready (≒domContentLoadedEventEnd) and window.onload (≒loadEventEnd) mesured by Navigation Timing (see browser compatibility).
Non onload blocking behavior
The 3rd party scripts are added to the queue that will be fetched immediately and excuted as soon as possible. It means "asynchronous loading and executing".
And you can see the timing of DOM ready is almost equal to the timing of onload (if you are not unlucky).
It means that the loading and excuting of 3rd party scripts doesn't block the onload for main contents, aka "non onload blocking".