To the folks downvoting, the following is from Facebook's Front End Tech Talk:

  /**
  * Adding a single line to this file requires great internal reflection
  * and thought. You must ask yourself if your one line addition is so
  * important, so critical to the success of the company, that it warrants
  * a slowdown for every user on every page load. Adding a single letter
  * here could cost thousands of man hours around the world.
  *
  * That is all.
  */

http://www.slideshare.net/makinde/javascript-primer Slide #47

That comment is in Primer which is a very small piece of JavaScript loaded at the beginning of every page.

Thanks, did FB ever release anything more from Primer? https://gist.github.com/376039

Or is this it? I didn't see the comment.

The Javelin JS framework used for Facebook Lite.

http://www.javelinjs.com/

That it is, it just excludes the comment at the header. It has actually changed a bit since then, but that is still pretty much it.