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This is what I found so far:
Aug 14 (Tue): WoW 11,220 players (48.4k hrs)
Aug 19: WoW 11,032 players (49.7k hrs)
Aug 26: GW2 10,488 players (79.6k hrs), WoW 11,222 players (48.9k hrs)
Sep 2: GW2 15,062 players (92.9k hrs), WoW 5,607 players (25k hrs)
Sep 9: GW2 14,091 players (79.4k hrs), WoW 5,675 players (25.4k hrs)
Sep 30: GW2 40.8k hrs, WoW 67.7k hrs (didn't find the player numbers, Xfire was being weird)
Oct 2 (Tue): GW2 7,914 players (28.7k hrs), WoW 9,473 players (52.1k hrs)
Aug 19-Aug 26: WoW 168k hrs
Aug 26-Sep 2: GW2 335.7k hrs, WoW 155k hrs
Sep 2-Sep 9: GW2 285.6k hrs, WoW 170k hrs
Sep 23-Sep 30: GW2 141.5k hrs, WoW 338k hrs
Yeah, I know that Xfire only measures Xfire players, that's why I put Raptr stats here too, so trends as well as differences across tools can be determined. To me, it looks like the trend has changed, in contrast to the early years when internet and the MMORPG genre were still growing, nowadays you'll see everyone flocking to a new MMORPG right at launch instead of over the years and after a while a core playerbase remains.
A trivia: unfortunately and annoyingly enough you can't count ingame server population in GW2, but around Aug 26 GW2 had 45 servers and a peak concurrent players figure of 400k players, which means a 9k concurrent players average per server (Rift had a player cap of 2k per server)