Or, at least, that's how many self-identified in their Twitter profiles as professional physicians in Emergency Medicine at the time this descriptive study was undertaken. According to the author estimates, this accounts for ~1.6% of the ~20,000 U.S. board-certified Emergency Physicians. The true number may be higher, owing to profiles that do not identify themselves professionally.
About half were "active" with a tweet within the last 15 days, and the other half were "inactive". Active accounts followed more users and were followed by more users. They also have a visualization figure showing the interconnectedness of the active Twitter accounts, and, unsurprisingly, everyone tweets to the same group of twits, and vice versa.
So, it's a small social media extension of the greater online presence of Emergency Physicians. I'd probably say that the primary flaw with the service, regarding promoting wider interaction between online EPs, is that it is a closed, self-contained system separate from the other online resources visited by EPs. The value is probably most to those who communicate and interact professionally in an active manner, whereas it doesn't have as much to offer the passive observer.
"Analysis of emergency physicians’ Twitter accounts"