We could just look at the investors who put significant sums in. Crunchbase has a list: http://www.crunchbase.com/company/zynga

To be fair to them all, it reads like a who's who of the VC world from Brad Feld and Reid Hoffman through to Andreessen Horowitz, Softbank, and Union Square Ventures. I'm not sure any blame could be laid at their doors though, it was an exciting and high growth company and that's exactly what they tend to invest in (and what gets celebrated the most around here on HN.. growth > profits, a "startup" isn't really a startup unless it's growing fast and all that, etc.)