Just released is the Q4-2014 Orr Report (Roll20), which details how many are playing what over the Roll20 gaming platform. Dark Heresy came in 8th on the list ...but it's a little more impressive than even that.
Essentially, there's all the D&D versions (including Pathfinder), all versions of Star Wars & then Dark Heresy ! Considering they've previously reported that Roll20 now, I believe, have north of 700,000 users, and gaining approximately 1,000 new users per day, it's a very reasonable sample size.
The report itself on Roll20's blog, which shows the entire list of games is here: http://blog.roll20.net/post/107957194710/the-orr-group-industry-report-q4-2014
Ave, Imperator