In a study of 3,128 HubSpot B2B customers in 2011, LinkedIn generated a visitor-to-lead conversion rate of 2.60%, on average. That's four times higher than the average visitor-to-lead rate for Twitter (0.67%) and seven times higher than the average rate for Facebook (0.39%).
Moreover, LinkedIn is growing faster than Twitter and Facebook
LinkedIn attracted some 28.1 million unique visitors in March 2012, up 7.3% from 26.2 million in February, and up 60.7% from 17.5 million one year earlier.
By contrast, Twitter attracted 42.0 million unique visitors Twitter in March, up 35.5% from 31.0 million visitors a year earlier. Facebook attracted 169.9 million visitors in March, up 20.7% from 140.7 million a year earlier.