If they were already public, this would likely cause a huge drop in the stock price. Similar to what happened to Netflix when they guided down on number of subscribers.
When you've sold yourself as a growth story, the reasons for the decline don't matter to Wall Street. Sell side analysts build complex earnings projections models based on estimated compounding growth rates. Even a slight reduction in the growth rate has a large impact on projected earnings a few years out and a negative growth rate blows up the entire model.
Did Zynga do anything to cause the user base to decline?