They need to hire a community manager (if they don't have one already).
Their job, is to simply manage the forums/facebook/twitter/youtube channel.
They should do things that ALL other games are currently doing, like have show off fan art, and stuff.
Also, this is how things work for a community manager:
Something happens in the game: (Such as a patch)
1. Make Youtube Video showing off new Patch. (If anything interesting was added)
2. Post on Twitter, with link to Patch Notes and Youtube Video.
3. Post on Facebook with link to Patch Notes and Youtube Video.
Fan sends in cool looking Fan Art or posts it on the forum.
1. Post fan art on Twitter, give recognition to the artist.
2. Make a "Fan Art" album on Facebook. Tag the artist if you know their Facebook Account.
3. If you start getting a lot of Fan Art, start doing weekly or bi-weekly Youtube Videos showing off your favorites and giving recognition to the artists.
Have an in-game deal? (Such as bonus Platinum, or Double-Exp weekend)
1. Post advertisement on Twitter. Ask people to tell their friends (great time to start the game)
2. Make post on Facebook.
Also, A lot of games have it so if you like their Facebook page you get something in-game.
I can go on and on, it is a full-time job to manage the community of a game. And it makes more money than you'd be paying the person you hire to do it. You would have SO many more players if you just had someone putting in work to advertise your game so people can FIND it.