Other fast-growing games on Facebook include Monster Galaxy (Gaia Online) with 14.7 million users (+17%), Army Attack (Digital Chocolate) with 2.6 million users (+208%) and PyramidVille (Kobojo) with 2.2 million users (+45%). It's a critical time for Zynga, which is reportedly preparing its initial public offering after a couple other successful social media IPOs. With Renren and LinkedIn happily out the gate and Facebook mulling an earlier-than-expected IPO with a $100 billion valuation, it's a pretty nice time for social startups to start trading publicly. To that end, Zynga needs to look like the best possible company right now, down to the fundamentals: that means keeping its user base enormous and always interested in the company's new games. Besides Empires & Allies, Zynga recently launched Hanging With Friends, a hangman-inspired game for the iPhone.