How many people get hit by celebratory gunshots into the air?

Nissemus tweets:

Wonder how many people have been hit by "celebratory" rounds fired into the air falling back to earth? #Libya

I've often wondered the same.  I think the number must be quite low.  Factors to consider:

• If fired at a steep angle, the bullet is in the air so long that it has time to bleed off excess velocity, and to reach terminal velocity on the way down.  While still dangerous, possibly lethal (depending what part of the body it hits), it is not as dangerous as a bullet fired straight at a living target.
• It will be very difficult to fire a round such that the projectile lands anywhere near the point of firing. If a large number of people celebrating a victory are gathered together, then a celebratory round fired from that crowd will be unlikely to hit anyone in that crowd.
• Outside of such a gathered crowd, the density of people outdoors will be comparatively low.  Manila has the highest population density at 43079/km²; if every last resident of Manila proper were outdoors, they would still only cover about 1/50th of the target surface area available to an unaimed bullet.
• Tripoli, specifically, is not as densely populated as Manila; Wikipedia notes a density of 4205/km².
• Let's allow that most Tripoli residents are outdoors during the celebrations.
• An upright human body might present about 0.5m² cross section for a falling bullet to hit.  Sometimes more, sometimes less.  Let's err on the side of caution.

Then, of every km² of land area, humans in Tripoli occupy at most about 2000m².  That is, 0.2%.  My guesstimate of an answer then to Nissemus' question would be that you might see one falling-bullet injury (not all of which would result in death) for every 500 rounds fired.