Although this video from Advertising Age is slightly dated (from August, 2009), it marks, roughly, the point at which Pfizer and other drug companies took the proverbial dive into one prominent pool of social media: Twitter. It seems at the beginning, though, Pfizer executives were unsure of what they could legally post on Twitter.
Since August, many other pharmaceutical companies have followed the path of Pfizer (and of, notably, Johnson and Johnson: @JNJComm being one of the first pharma Twitter accounts, created in February, 2009, and currently boasting a total of 3,136 followers. By anecdotal comparison, George Clooney only has 1,330 followers. And for some reason, @GSKUS (Glaxo Smith Kline's Twitter account) is following Ashton Kutcher and Mandy Moore.).