According to this article in today’s AdWeek, it’s BarbieGirls (an August 13th BG press release states they’ve accumulated 4-million users in the first 90 days and are now adding 45,000 new users a day. It took SL three years to top the 1-million mark, and it’s now closing in on 9-million residents).

Visit the site and you’ll see a video of how membership works – you need to buy a Barbie Girl figure (which looks like it might just be a fancy USB key) that you attach via a dock to your PC in order to get access.
Naturally, concern #1 for most parents would be the pedophile factor, and of course purchase of Barbie Girls is not limited to, well, girls. According to the “Message to Parents” page, there are filters in place that limit words and phrases that can be used and all kinds of features that enable the kids using the site to block weirdos.
Girls being girls, however, I think the biggest thing we’re going to be hearing about once school starts is “virtual bullying” on Barbie Girls. You know, throwing a party online and inviting everyone in the class but Cyenna, etc.
I’d probably have to buy a Barbie Girl to find out, but I wonder – do the community managers offer advice like that painted on the cars driven by the Dearborn Police (“Be nice to people”)? They should – it’s a pretty important lesson.