24% of twitter users review or rate products

"Interpret surveyed over 9,200 internet users in August, finding that roughly 24 percent of the respondents that used Twitter, reviewed or rated products online; just 12 percent of people that used other social nets—but not Twitter—said the same. Twitter users were also more likely to visit company profiles (20 percent) than non-Twitter users (11 percent), and twice as likely to click on ads or sponsored links (20 percent vs. 9 percent).
The higher rates of engagement with brands could stem from a number of factors. First, people aren’t spending as much time doing activities (commenting on photos, chatting, playing games) on Twitter, so they could be more apt to click on an ad that would take them away from the site. In contrast, it’s been proven that most ads on Facebook, MySpace and other social networks, fail to garner high click-throughs or other interactions, simply because users don’t want to have their experience interrupted."
Source: Interpret research, as reported by PaidContent, 24th September 2009