Looks like a bug. There should be a number greater than 0.
The point of it is that players are given statistical information to do with what they want. It means that you know how many people are playing online.
For example, in SC2, when logged into Battle.Net, you're given some population numbers missing from D3, such as:
About 500,000 players on Battle.Net
8643 StarCraft II games worldwide
4627 StarCraft II games in your region
About 970 players in public channels
Otherwise, the only visible D3 player population numbers are the Public Games and Players On This Quest. And those numbers give the public a sense (or enforce) the idea that the game is dying (notice the descending, constant trend):
Number of Available Public Games (games you can join) in The Americas Region
Sundays, 8:30pm PST
6/24/12 = 2072
7/1/12 = 1832
7/8/12 = 1649
7/15/12 = 1351
7/22/12 = 1217
7/29/12 = 1061
8/12/12 = 877
8/19/12 = 837
8/26/12 = 952
9/2/12 = 931
9/9/12 = 802
9/16/12 = 860
9/23/12 = 710
9/30/12 = 619
10/7/12 = 639
10/14/12 = 624
10/21/12 = 546
10/28/12 = 497
11/4/12 = 457
11/11/12 = 471
11/25/12 = 461
12/2/12 = 448
12/30/12 = 555
1/6/13 = 420
1/13/13 = 356
Edited by Nabs#1616 on 1/20/2013 8:53 PM PST
Public game counts are meaningless since they changed it so many times. Now that it matches much better to avoid unfull games you don't see nearly as many open public games even though there has been an increase in players recently.
Of course now with monster power in public games the counts are even lower since people are spread out even more then before.
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