Social Media Benchmarks
I don't think you'll find a meaningful number for this unless you are looking for a somewhat narrowly defined component of social media. Social media programs as we define them rely on a wide range of variables. A PR firm trying to reach out to top influential bloggers for example, may only be a single component to a program with several other tactics.
Although a cool new blogger and public relations study may tell me something like 42% of A-list bloggers who are pitched on something from a PR professional will actually write about it - I'm left guessing about fair traffic expectations (let alone conversions) generated from those blog sites.
