I sort of chalked it up an an anomaly. In all, GA shows 474 visits from facebook.com. I paid for 543 clicks. I suppose people could have hit stop/escape after

I sort of chalked it up an an anomaly. In all, GA shows 474 visits from facebook.com. I paid for 543 clicks. I suppose people could have hit stop/escape after the click was registered by FB but before the page loaded and the javascript registered.