Being a certified geek myself (yes, my Christmas list was on ThinkGeek.com), I remember those previous attempts at building communities online. They didn't work for a variety of reasons, the biggest of which was the lack of a rich interface, and really slow connections. Now that Facebook has solved the problem with the rich interface and broadband providers have solved the one about slow connections, Facebook has what it needs to be a success.
But there are other organizations out there that have strived to be communities like Facebook and haven't succeeded. I think that what makes Facebook different is that it provides great flexibility and it gives users the appearance of control. You can choose your friends, you can ignore people you don't like, you can post videos and photos, and you can create groups for like-minded people (I created one for the local NTRAK model railroading community).
But Facebook is about more than just recreation. Now, it's an important way to connect to your customer or user community. It's a broadly accepted path between your sales team and your potential customers. We're reaching the point where the lack of presence by your company on Facebook may say something about your company that you may not want said.
As a result, Facebook has insinuated itself into all aspects of your customers' lives, and perhaps even your company's life. It's a vital communications tool that may be very new, but is nevertheless very important. And of course it is this impact on the day-to-day lives of people around the world that got the attention of the editors of Time. This year, at least, Mark Zuckerberg and Facebook really did affect more people than anyone else.