Like its overgrown canine namesake, social gaming company Zynga has become the largest developer on Facebook’s platform over the past year. And, it cannot get away from whispers about its plans for an initial public offering. A key reason: Within the last six months, it has piled on a series of hit games, from virtual poker game Texas Hold ‘Em to mob-themed role-playing game Mafia Wars to virtual world YoVille — and most recently, FarmVille, which just hit 50 million monthly active users this past week.

It is proving the possibility of social gaming companies being long-term hit factories, not unlike more traditional, publicly-traded gaming companies like EA. And it is, from what we and others here, bringing in the money to prove that it is a real business.

So what’s going on with the IPO plan? This spring, a reliable industry source insisted that Zynga was in the process of closing a round of funding for possibly around $30 million. Another industry source now adds that it had at that point hired Allen & Co., the boutique investment bank, to raise a round of funding with a valuation between $550 million and $600 million. But when we asked about the funding, then, the company adamantly denied that it had raised any new round.

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