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.

Some other trends worth noting:

– LinkedIn grew 146% year-over-year to 10.8 million unique visitors

– Imeem fell from 3.9 million unique visitors in July to 3.4 million in August. They could be one of the more impacted companies from this month’s expected launch of MySpace Music.

– Buzznet traffic fell 54% year-over-year to 1.8 million unique visitors. The company has recently made a number of acquisitions including Idolator and Qloud to bolster its audience.