40k still has the largest online following from what I can tell. As for an exact count of any kind, I couldn't say but I'd say GW still holds most of the miniature wargaming market and of that 40k is likely the biggest.
Here it is probably the 4th most popular mainly thanks to a pretty awesome new FLGS opening and GW stopping all gaming nights. There is one last 40k club which gets good numbers, but it's lost a lot it's members to the FLGS and other games.
Malifaux and Infinity are the most popular by a long long stretch. 3rd place i'd say is Infected Dead or Warmachine.
Worldwide I expect 40k is the most popular still by quite a long margin as it is the game that most wargamers start out with.
Have to say it is difficult to say though because with many other gaming systems you only have maybe 5-8 models per faction and that will set you back about £25-£30. Then maybe another £25.00 for the rulebook, which is optional in games like Malifaux because the rules are available for free online via the website. Then there are dice and a deck of cards and occasionally updated stats but all of that costs no more than £10.00. So basically to play games like Infinity and Malifaux you would typically spend about £50.00. With 40k you need about x10 times more models depending on army, and need to spend about £100 for basic rules and codex, more for supplements and rules like Escalation. So it's impossible to know just by looking at sales figures.
Its one of those questions I don't think we will know the answer too. I think 40k is a game people come back too after trying other games out. Its availability of opponents is the reason it is so popular. Plus the deals you can still get online to reduce the rice to normal.
I am constantly hearingabout people going to give up 40 due to GW pissing them off, but I very rarely see it happening.
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