Well thats an old number, there have been about 4500 registrations. I have spent roughly $80-$100 on advertising. The traffic comes from voting sites surprisingly, as a game owner you need to check the sites that will give your game the best exposure and reward your members for voting.
Find a site that accepts articles and write a very good one that generates excitement for your game. One article i wrote got in excess of 1200 views and it referred literally hundreds of players.
Post your game on forums to people who havent tried browser games yet as these are most likely to stay.
Make your game as interesting and as unique as possible.And thats basically it.