This is very hard to get exactly right, because traffic isn't constant at all times, and you don't know if people won't just make up for lost time using facebook at another time of the day, etc. So you can't really know.

But, a good rule of thumb right now is about $10,000,000 per hour.

Facebook made $29 billion last quarter which translates to $315,217,391 per day. Divide that over 24 hours in a day, and it's ~$13 million per hour.

Of course, depends on the hour of day. Facebook likely makes more ad money when North America is awake than when Asia is awake for instance.