This recent poll (ended 13th February 2017) on TMP shows that 34 out of 140 wargamers played FOW in the past year.
That is 24% which could easily be the most popular historical wargame.
http://theminiaturespage.com/polls/303905334/
I dunno, they're asking wargamers how many games of FoW they played over the last year, which doesn't invite any sort of comparison with other games. 24% said they'd played at least one game of it, but only 9% said they'd played more than 5 games. 76% said they hadn't played it at all! Without knowing how many other systems were played, or how the players liked the system, I think broad interpretation of the results is difficult to say the least. It could be the most popular ... or not. The poll doesn't really address that.