Prepaid game cards are becoming an increasingly popular way for players to buy virtual currency in MMOs and virtual worlds, and they may soon become a popular way for players of social games on Facebook and MySpace.

Zynga, the largest developer of Facebook and MySpace games — and who we hear is nearing $200 million in annualized revenue — may soon launch a series of prepaid cards with leading provider InComm, we hear. The cards would enable players to purchase credit toward virtual currency in popular Zynga games like Texas HoldEm Poker, YoVille, FarmVille, and more.

InComm, whose cards are available in 145,000 retail stores worldwide, could help boost Zynga’s virtual currency sales in particular with players who don’t have access to a credit card. Prepaid cards are especially popular with younger players who may not be able to bill significant purchases to their parents’ credit cards and who may not be eligible to participate in other alternative payment methods like offers. They are often sold at cash registers in game, grocery, electronics, and convenience stores.

Earlier this year, InComm reported that their game currency card business grew 200% over early 2008, and has been launching new gaming cards throughout the year with partners like Cartoon Network and Ndoors. Adding a partner like Zynga could drive InComm’s business even faster this year.