The best health metric used for massively multiplayer online (MMO) games these days happens to be how many people play them in a given period—often a month—but another overlooked standard is the total number of registered users. Sony Online’s Free Realms MMO has hit a milestone of over 20 million registered users since its launch in spring 2009.

This may be a wonderful milestone, but it’s still not as useful as the monthly user count. The reason is because Free Realms happens to be a free-to-play MMO, which means that users can fire-and-forget their accounts and it will build their registered user base. Subscription-based MMOs, such as World of Warcraft and RIFT both continue to charge participants whether they play the game or not (thus generating revenue even if players do not participate that month.) Free-to-play MMOs depend instead on users purchasing from their virtual item shops.

As a result, while 20 million registered users sounds extremely daunting, how many of those users are active during a month would probably be a better indicator.

The game is available on PC, Mac, and PlayStation Network and features a fantasy-based MMO experience where players are given a chance to involve themselves in adventures, minigames, and other social activities. Massively multiplayer games function by bringing people together to overcome challenges—often the requirements of quests and activities involve other players—and as a result this works to glue players to the returning to the game again and again.

Companies such as Blizzard—who’s BlizzCon convention begins this very day—use the social factor to keep subscribers; free-to-play games like Free Realms use it instead to entice users to purchase virtual items from the cash shop. Virtual items often work to enhance gameplay, change player avatar features (such as hats, outfits, hair-styles, etc.) and sometimes also include boons that give them an advantage over other players in cooperative and competitive venues.

Even the subscriber-based models such as World of Warcraft have experimented with virtual items—often to huge profits—as it’s obvious that social gamers are more likely to shell out coin and are slowing changing the landscape of gaming.

The president of Sony Online Entertainment, John Smedley, says the benchmark is a testament to the company’s success with the free-to-play model (vs. subscription base); but as I said above, free-to-play just allows would-be players to fire-and-forget and really this just shows how well advertised their game is. The monthly active players will be a much better metric as to exactly how healthy their game is.

Still, 20 million registered users is nothing to sneeze at—as long as each of them has on average bought a $1 item from the cash shop, that would be a lot of revenue.