Dell have generated $4.5 million in revenue through twitter in the last 6 months

"It's been widely reported by now that Dell's @delloutlet Twitter account that broadcasts deals on computer equipment has earned the company $6.5 million. The more interesting part, and the one that's overlooked, is that $4.5 million of this money was earned between June and December. In June, the company said the account had brought in just $2M since the its launch two years earlier (or, they say, $2M in outlet items and another $1M in new products, but they don't break down the $6.5M number). Last December, the amount was at $1M.
So, it took Dell 18 months to earn its first Twitter million. Then it took just six months to add another million to that. And the next six months brought in $4.5M."
Source: Adverlab, 10th December 2009
Additional info:
DellOutlet's followers on twitter have risen from 208,864 on 23rd March 2009 to 1,477,440 on 11th December, according to Twitterholic, retrieved 11th December 2009