Zynga is the darling of social games, but it’s hard to believe that the maker of Facebook games is worth $5 billion, as estimated by SecondShares.com, which is run by a group of former equity analysts who evaluate the value of private companies and what they would be worth if they were actually public.

While Zynga has no near-term plans to go public, the analysts can surmise its value based on a variety of measures, such as the price that employees are getting by selling their stock options on the secondary market. There is a lot of black art to this kind of evaluation, and the high value for Zynga is likely to stir a lot of debate.

It’s hard to watch some of this from the sidelines, sort of like watching the craze over the dotcom madness of the late 1990s. Yet there are differences between a company like Zynga, which is profitable, and Pets.com, which had no hope of being a money maker. There is a gold rush mentality in social gaming. You can imagine that executives at Microsoft, Google, and Facebook would look at this report and conclude, “Oh dear, we have to buy them now or else they’re going to get even more expensive.” Crazy valuations have a way of inspiring even more crazy behavior.

My guess is that the top folks at Zynga don’t like some of this speculation about Zynga’s imaginary public value. They, of course, stand to get rich at some point from Zynga’s high valuation. But they don’t want to be over-hyped, only to fail to meet expectations. Getting a valuation like this is kind of a jinx. Mark Pincus, chief executive, said that after his company was profiled in Fortune magazine. Tonight, Pincus offered no comment when asked about the estimate via email.