Most of the momentum in the virtual goods market comes not from gifts but from social games, where people buy items to improve their performance in the game or just to build up a collection that will impress friends.

Unlike traditional games, social games are generally free, and the vast majority of players never spend any money. In Zynga’s games, less than 3 percent of players pay for something, said Mark Pincus, the chief executive of the company.

Players can also earn virtual currency by signing up for subscription services or installing pop-up advertising software. But some social gaming companies have cut back on such offers after criticism that they were misleading and in some cases defrauding players.

Zynga says direct purchases of virtual currency and goods will account for most of its more than $100 million in revenue this year, and that the company is profitable.

Game creators talk openly about their strategies to make people pay for virtual goods: get them addicted, then steer them to purchases that speed up the pace of the game and help them succeed. In FarmVille, for example, the tractors’ gasoline tanks replenish themselves slowly over the course of a day. Instead of waiting, players can pay to buy gas  something that might be considered cheating in more traditional games.

“You put intentional friction in, and a small number of people who value their time and want to play at a faster pace can spend money,” Mr. Pincus said.

Players of the games have competitive reasons to buy, too. Wendy Pickering of Columbus, Ohio, plays Sorority Life, a game in which players create and dress groups of co-eds, and then, rather violently, pit them against one another until the most glamorous house wins. She discovered very quickly, she said, that she would be trounced in every showdown if she didn’t have enough fashionable items.