gy start-ups to cash in their shares are establishing rough valuations for hot companies. Zynga Inc., the San Francisco company behind online games FarmVille and FrontierVille, is valued at $5.51 billion, according to SharesPost Inc. The private exchanges sprang up partly because the market for initial p

industry growing to $60 billion from $30 billion five years ago. And the fastest growing major videogame company is Zynga (maker of games including FarmVille), which delivers its games entirely online. Zynga's first- quarter revenues grew to $235 million this year. more than double revenues from a year ear