According to this MSNBC article, there are no less than 200 youth-oriented virtual worlds and MMOs either in development or on the market. Some of these titles even boast account numbers that rival Facebook and MySpace as analysts estimate 30M users in Club Penguin, 52M users in Neopets, and 90M in Habbo.
Is this really surprising though? Probably not when you consider that kids today aren't totally different in this generation than previous generations. 'Tweens have always liked to socialize, play, and simply "hang out." The difference nowadays is that parents tend to have looser restrictions and the kids can accomplish most of the things they like to do without even leaving a computer chair. The rules and tools have changed.
The article also talks about a 89/10/1 rule that most F2P/RMT games aim for. This rule assumes that 89% of people will play your game for free, 10% will pay the minimum, and 1% will spend lots of money. Apply that rule to the recent announcements that both Free Realms and Runes of Magic have surpassed 1M users. If the rule applies, it means 100,000 people are paying the basic fees (~$5/month) and 10,000 are spending oodles on extras ($15+/month). That's roughly $650,000 per month, which isn't half bad depending on your development costs.