PR (Public Relations) is a free way to promote your online game. Local press are the folks who first pick up news from the area and are most likely to publish the news. The regional and/or national press would only pick up the story after that. You should write up a short announcement of the release of your online game and then call your local press. Be prepared to tell them why your news will be of interest to their readers (that is all that they care about) and also be prepared to help them write up the story (by providing the written announcement, screenshots, and anything else that they need.) The key issue to to convince them that your story is newsworthy in that their readers will find it valuable. Free press like this can get you a lot of exposure (i.e., traffic.)
You can choose from many marketing method such as forum marketing, social media marketing, blogging, email marketing. Find what works for you. I'm sure, you will be able to find some techniques that would work for you. Goodluck.
Social media can be a great way to market your game. The other thing you may want to consider is using bloggers to help spread the word. Find gaming bloggers to write reviews for and share your product with there audience. Bloggers have influence and can become your own little brand ambassadors! Good luck!