So what happened? Shortly put, Zynga's stock wasn't really worth what it was priced out at. An initial public offering is designed to be priced a little low, in order to drum up demand for a company's stock from the public (not to mention raise some money). But Zynga went high and, as a result, didn't quite get the graph it wanted today.
They didn't sell the FarmVille, so to speak -- ZNGA will likely be trading fine on Monday (and $9.50 is fine for the highly competitive gaming industry; THQ is sitting down at 75 cents right now). But Zynga's hype phase appears to be over. Now the company needs to prove it can sell more than just cow clickers.