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A small new survey from Nielsen about the five fastest growing "member community destinations" in the U.S. reveals what we all kind of knew already: Twitter is at the top. From February 2008 to February 2009, it clocked in at a whopping 1,382 percent growth rate. That's to be expected, considering the amount of press the still-without-a-business-model microblogging service has gotten in recent months.

In third place is Facebook, with 228 percent growth year-over-year according to Nielsen. That's not terribly surprising, as Facebook is still growing in the U.S. but not quite as exponentially as it once was.nick.com/kca

There are, beyond that, a handful of interesting things to note. Two of Nielsen's top five, for example, aren't social networks but rather wiki creation services: Zimbio (240 percent growth) and Wikia (172 percent growth). And in fourth place is Multiply, which probably got a surge of activity when it recently acquired the MSN Groups service that Microsoft was spinning off.

But a blog post from Nielsen said that Twitter (which counts the 35-to-49 age demographic as its biggest, the statistics said) may be growing even faster than its numbers say. "PC Web usage of Twitter.com doesn't tell the whole story," the post by Nielsen Online's Michelle McGiboney read. "The ability to (use) Twitter via a mobile phone--whether through the mobile Web or via text messages--is a driving factor in the social network's success. In January, 735,000 unique visitors accessed the Twitter Web site through their mobile phones. The average unique visitor went to Twitter.com 14 times during the month and spent an average of seven minutes on the site."

An additional 812,000 users accessed Twitter via text message on the AT&T and Verizon carriers alone.