Could be a browser extension; FB app you gave access to; malware; any number of reasons a website could access audio devices on your machine.
It was 6am and I had my earbuds in and I could tell they turned on. They didn't turn off until a few minutes after I logged off.
I went to YouTube just to check. A few minutes after I closed my YouTube tab, I could hear the audio disengage.
I'm wondering if it's connected to FB autoplay videos, but those have no sound.
(same setup - current Chrome / current OS X)
I turned on auto play videos, killed the browser, waited for my headphones to turn off and went back into FB - scrolled down and when a video came into view, the headphones turned on.
Even though the video might have no sound, the html 5 video element still needs to allow sound to pass through. When videos auto play without side, it's not that the video doesn't have sound; it's just that the video has been muted until you click on it.
(speculation of course; I don't work for FB)