PLAYERS of FarmVille, an online game, raise virtual chickens on an imaginary farm. Yet they are happy to swap real money for virtual money to buy virtual farm tools. And investors are likely to pay more than chicken feed for shares in Zynga, the firm that makes FarmVille and other online games.

Zynga is expected to file soon for an initial public offering (IPO). Analysts predict that the firm will be valued at between $15 billion and $20 billion. That is about as much as the world's two biggest video-game makers (Electronic Arts and Activision Blizzard) combined.

More than 271m people play Zynga's games at least once a month, and the firm said in March that it expects to make a profit this year of $630m on revenues of $1.8 billion. So its business is more real than those of some other online firms. But it is not something a sober investor would bet the farm on. Users may tire of virtual vegetables and online Mafia Wars (another popular Zynga game). Rivals are straining to grab Zynga's players. Electronic Arts, Playdom and Wooga have only about 30m monthly active users each, but they may catch up.

What is more, Zynga depends on two other firms, Amazon and Facebook, like the cabbage crop depends on the rain. Although it operates data centres of its own, it outsources much of its computing to Amazon Web Services, the cloud-computing arm of the online shopping giant. More importantly, most users play Zynga's games on Facebook. In September the social network pushed Zynga into using its virtual currency, called “Facebook Credits”, so Facebook gets 30% of what Zynga's users spend.

With Zynga gearing up for its IPO, the question now is which other tech start-up is next in line to go public. With Groupon, an online coupon service, also about to float, the supply of hot stocks is running low.

But Silicon Valley venture capitalists are busy replenishing the pool. On June 24th it emerged that foursquare, a location-based service, had raised $50m, a deal that values it at $600m. (Foursquare lets users electronically “check in” at bars and restaurants so their friends can join them—and the people who owe them money can avoid them.) A few days later investors pumped $100m into Square, a mobile-payments start-up, valuing it at $1 billion. Neither firm has ever turned a profit.