Nearly eight out of ten Global 100 companies are using some kind of social media, according to a new study (which I'm afraid is in annoying pdf format and not an easy read). For the companies that actually use Twitter -- and it's the most popular social networking site for businesses -- they're very active. Facebook is second, followed by YouTube and blogs.
In terms of what people know, the corporate shift to social networking means they're reaching out directly to people with their message. Whether this outreach matters is, to me, unclear. Not much data out there to answer that one.