Your acquired users are your most valuable commodity. They were hard to come by, and if kept happy, they will spread the word, attracting more users.
Needless to say, you need a fun game. So what else can be done to keep your users playing?
Update as often as possible, every two weeks if you can. Plan future additions and features even before you release your game. Hold back some features if you must, but have a plan.
If you update often, your players will feel that you are dedicated to the game, and in return will reward you by sticking around.
If you monetize with ads, be careful not to annoy. Use ads sparingly.
Differentiate between your loyal users and treat them with personalized messages or perks. These players will feel important and most likely stick with your game for a long time.
Give users the ability to leave feedback from inside the game. Incorporate the feedback on your website or Facebook page. Comment and discuss the issues, the dialogue will make the users feel invested in your game. But, do follow up on sensible suggestions with regular updates.
Allow your users to compete. Even better, make leader-boards regional or filtered by skill. It is much more motivating to try and creep up the leader-board from place 50 than from place 3000.
Allow them to invite friends. It will grow your user base, but more importantly, if your users have more friends playing the same game, they are more likely to stay.
Facebook, Twitter, Email are the minimum.