Well, that was fast. It was just Friday that multiple law firms publicly announced investigations into allegations that Zynga executives and underwriters had illegally profited from insider information, selling off their stock well ahead of last week's dismal earnings report. Now, San Francisco's Newman Ferrarra has become the first firm to actually file a class-action lawsuit in the matter, laying out a specific sequence of events it says points to an insider trading scheme.

The full complaint [PDF], filed in Northern California District Court, notes that Zynga's executives agreed in an initial prospectus to temporarily "lock-up" their substantial holdings in the company, pledging not to sell or transfer shares until May 28, 2012, 165 days after the IPO. But Zynga amended that agreement with a waiver issued on March 23, allowing executives and underwriting stockholders to sell nearly 50 million shares much earlier than initially promised. Those parties managed to pocket around $500 million by selling at $12 a share in early April, thanks to that waiver.

Meanwhile, Zynga continued to project an image of expected growth to the outside world, projecting as recently as April that 2012 revenue would come in at $1.425 to $1.5 billion for the year. After last week's quarterly earnings report came in well below the expectations, Zynga abruptly lowered that outlook to a range of $1.15 to $1.23 billion for the full year, helping to send the stock price tumbling down to around $3 a share (a full 75 percent lower than the sales price under the waiver). The new guidance also "directly contradicted management’s prior comments about expected bookings growth that Zynga’s bookings would be weighted more heavily towards the second half of 2012," as the lawsuit puts it.

The lawsuit quotes analysts that were shocked by the poor results as proof that Zynga was withholding crucial information about the company's ill health, while at the same time tweaking previously set rules so certain stockholders could sell off before the coming revelations. Specifically, the lawsuit calls out Zynga for failing to tell stockholders that player numbers and virtual goods sales for its most profitable games were dropping substantially, and that some of its new games were going to launch later than previously expected.

These are serious allegations, and ones that are likely to cause major legal problems for Zynga at a time when many are beginning to doubt the long-term viability of its microtransaction-heavy business model. If social gaming's biggest ship truly is starting to sink, you can expect more stockholders will soon join in an effort to extract what value they can before things end up fully underwater.