After it announced layoffs of 520 employees and lowered its guidance to Wall Street, Zynga’s stock dropped precipitously yesterday, down 12 percent, to dip below $3 a share. The decline put the market value of the company at just $2.34 billion, well below the once much-hyped hopes for the San Francisco-based online gaming company that has seen nothing but troubled times since its IPO. In fact, since its late 2011 public offering, Zynga shares are down close to 70 percent. But perhaps more interesting is that, with $1.6 billion in cash and marketable securities, investors now consider the company to be worth just below $750 million. In other words, about $350 million less than Yahoo just paid for the blogging platform Tumblr, which has substantively less revenue than Zynga.