Micro-blogging site Twitter has grown almost fourfold from 533,000 to 2.4 million visitors.

Among more established social networks, LinkedIn was the fastest-growing--nearly tripling its audience to 11.9 million. That growth rate helped the site for professionals this week close another $22.7 million in venture capital. In June, it raised $53 million.

Facebook continued to grow at a healthy clip, more than doubling its traffic from 18 million a year ago to 39 million. MySpace remained the largest social network with an audience of 59.3 million, but its traffic has been essentially flat, increasing only 1%.--Mark Walsh

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