I wouldn't say FH is "becoming unpopular" as such, but I can see how some may think that.
It's most likely just because of the school year coming to a close for most people, I have a feeling the game as well as the forum will become more active when the summer holidays finally start. Like Suragaha said... T'is the season for exams.
And as for the forum situation... Most of the community don't even post on the forum... If they're on here, it's probably because they are looking for a map download or something. I doubt the forum will ever be as busy as in-game, no matter what time of the year it is. Why? Simply because most members - especially new ones - really don't have a reason (other than that) to be here. The game's download links have their own page entirely, so looking through forum threads isn't necessary. After that they're off and into the game and chances are they won't be back on the website for a while.