Play No Evil offers up a good article breaking down why Free To Play is a superior engagement model to subscription or retail. Matt Mihaly from Iron Realms jumps in to offer a counterpoint on MapleStory's user base (i.e. 57M is a gross number, not representative of active users) and offer up Habbo Hotel as another North American virtual item sales success story.
In regard to Maple Story's user base, all 57M of them may not be active, but most of them are probably unique due to the fact that in China and South Korea, Maple Story requires users to enter their Social Security Number to sign up. So there likely aren't very many duplicates, world-wide.
Regarding Habbo's numbers, Paul Thind (Habbo's GM) reported their active uniques at 7.5M world-wide (1.7M uniques/month) at last month's Virtual Goods Summit. Habbo's annual revenues sit north of $65M.
From the article on Play No Evil, I especially like this passage:
The choice of prices are important. Buying blocks of currency or subscriptions for less than $10 means that it is an impulse buy. $50, or even $20, to buy a game is above the "whine factor" for parents. Nexon's ability to get payment cards into Target (see previous article) was critical - online game play becomes an impulse purchase at the checkout line. I think this is an area where many casual game companies "don't get it". Their prices are above the impulse purchase level.
In a related story, Fox News - that bastion of sound journalism - just did a piece on MapleStory and game addiction.