Just found this on the same site:
August 9th, 2015: 824,000 active accounts across all servers NA/EU/JP http://luckybancho.ldblog.jp/archives/45050121.html
April 2015: 730,000 active accounts http://luckybancho.ldblog.jp/archives/43689452.html
December 2014: 574,000 active accounts http://luckybancho.ldblog.jp/archives/41962303.html
September 2014: 650,000 active accounts http://luckybancho.ldblog.jp/archives/40214066.html
As is to be expected the number of active players seems to rise with each patch and goes down in the time between patches. The overall trend is upwards though.
Edit: for those interested...
Launch 2.0 -- Tuesday, Aug 27th, 2013
Patch 2.1 -- Tuesday, Dec 17th, 2013 -- 112 days after launch
Patch 2.2 -- Thursday, March 27th, 2014 -- 101 days after 2.1
Patch 2.3 -- Tuesday, Jul 8th, 2014 -- 103 days after 2.2
Patch 2.4 -- Tuesday, Oct 28th, 2014 -- 113 days after 2.3
Patch 2.5 -- Tuesday Jan 20th, 2015 -- 84 days after patch 2.4 (several weeks early, in other words)
3.0 -- Tuesday, Jun 23rd, 2015 -- 154 days after patch 2.5 (238 days after 2.4. Time between 2.0 and 2.2 was 212 days. The time between 2.2 and 2.4 was 215 days)
So the average has been roughly 111 days which is something like 3 months and 18 days. This means that patch 3.1 is likely to be released on Tuesday, Oct 13th, 2015 (112 days after 3.0 officially launched).
TL;DR: So it seems that the number of active players was at 650,000 before the expansion was officially announced in October 2014. It went down to 574,000 during patch 2.4 (a coil patch and just after the release of Warlords of Draenor) roughly a month before 2.5 was released. It then stayed relatively high at 730,000 all the way until April 2015. Now in August we're at 824,000 active players and we've got roughly 50 days until patch 3.1.
The data collection isn't perfect, so the results are likely inaccurate, but it's the best guess we can make at this point.