Last night, Nielsen Online reported that Twitter has now surpassed Facebook and others to become the fastest-growing site in the "Member Communities" category for the month of February. Although Facebook, the world's most popular social network, has more members than Twitter, that's not what this measurement is about - it's about growth. And Twitter is growing. It's growing like crazy.

Twitter's Growth

According to a post on Nielsen Wire, unique visitors to Twitter.com increased 1,382 percent year over year, from 475,000 unique visitors in February 2008 to 7 million in February 2009. This growth earned it the title of "fastest growing member community site," a term that encompasses not just social networks but any online community - even one such as online wiki community Wikia, which, incidentally, came in at number five on the list. Zimbio and Facebook followed Twitter, growing 240 percent and 228 percent, respectively.

Nielsen also reported that the largest age group on Twitter was not college students or teens, but adults from the ages of 35-49. This group comprises nearly 42% of the site's audience at 3 million unique visitors. Twitter is also a popular site for people to visit while at work, notes Nielsen's Michelle McGiboney, as 62% of the combo unique audience accesses Twitter.com from work versus only 35% from home.

Of course, visitor stats to Twitter.com from a traditional web browser don't show a complete picture since the service is also accessible from a number of desktop and mobile clients as well as SMS. In January, 735,000 unique visitors hit Twitter's mobile web site, averaging 14 visits per month and spending an average of 7 minutes per visit. Twitter also had 812,000 unique users sending and receiving text messages in the last quarter of 2008. However, this last stat only took into account AT&T and Verizon cell phones. Within that group, though, there were nearly 240 tweets per person for the quarter.

A Word About These Numbers

Before taking these numbers to the bank, it's worth noting that they are being pulled from Nielsen NetView (U.S. Home & Work), so they're not representative of the service as a whole - they only give us a snapshot of what's occurring there. Also, the demographic chart is annotated with a note that reads "these demographics have insufficient sample sizes" in the 18-24 age group column, which may speak to the overall insufficient sample size of this particular survey, a number which was not reported.

That said, these numbers do seem to confirm what our gut instincts have been telling us for some time. Twitter is apparently not becoming the next big thing with teens and other members of Gen Y, despite rumors to the contrary. The hype surrounding Twitter's connection to the younger generation was even skewered hilariously by Jon Stewart not too long ago on "The Daily Show."

It looks like Twitter may be for "old people" after all. Just like Facebook.