A couple people did a /WHO command and took some averages, and based on some basic math, it seems each zone is capped at 50 players per shard.
I personally have never seen more than 9 or 10 at any one time.
This is just very rough Guesstimation though. Blizzard never tells anyone anything, but yea, the world feels very much dead with population. Something is defiantly askew.