I don't think they are publicly traded. Their websites "investors" links lists which companies are their major investors, companies like Time Warner, NBC Universal, and four different venture capital firm. You might want to try contacting one of the venture capital firms to see if you can get a piece of the action but until it IPO's you're pretty much out of the picture unless you have a few million burning a hole in your pocket.
Keep in mind that with online games, it's really not anything new technologically so it's all about content, media and promotion. A game can be a stellar game and still fail in that market, less than 20% of such games make it, the difference being if the content captures the publics imagination and the promotion was sufficient to give it exposure. Also, the Internet is quickly becoming niche monopolies, Amazon dominates books and general merchandise, Apple dominates music, Google dominates the search engine, Twitter dominates whatever twitter is, and what dominates the online gaming community is Farmville. It doesn't necessarily take a fantastic game to dominate the market, just have the gamemification and psychology hooking the public.
That pretty much means that 80% of such games are a bad investment, this means that if by explode you are expecting a ten fold return on your money then you should invest no more than 11% of your portfolio in such a venture to maximize the geometric mean of outcomes for optimal capital growth in the long run. Of course, that's a simplification that only considers a 20% chance of a 10 fold return and a 80% of a total loss of investment but the point is that you're better off looking for higher probability returns.