The latest numbers are in, and Twitter is apparently growing at a torrid pace. According to stats just released from Nielsen Online, Twitter recorded 2.3 million unique visitors in August (US-only), an increase of 422% from the same period last year.

Moreover, visitors to Twitter spent 55% more time on the site on average ? a total of more than 7 minutes per user. Those numbers point to rather robust growth for the site, especially considering many of its most rabid users access it through a third-party client like Twhirl or Tweetdeck.

Elsewhere in the social networking space, Facebook continued to narrow the gap on MySpace, a trend we also noticed last month. Unique visitors to the site grew to 38.2 million, representing a more than 10% month-over-month increase and a 100% jump year-over-year. MySpace saw 61.3 million unique visitors ? up slightly from last month, but still essentially flat compared to the same period last year.