I hate to go Clinton on you, but a big part of this obviously depends on your operational definition for "used."
That said, I did some quick back-of-the-envelope calculations. There are approximately 25 250 registered users on the SDN forums. According to MSAR, there were 37 364 applicants to medical schools for the 2005 entering year. If we multiply that number by 4, we get just under 150 000 undergraduate "pre-meds" at any one time. Given just these numbers, we get that approximately 17% of pre-meds "use" SDN. I would take that figure to be a maximum estimate.
Now the numbers get fun. Looking at the number of posts in the various forums, pre-medical (MD/DO) topics are approximately 60% of the traffic on the forums. From this, we can infer that approximately 60% of the SDN members are currently "pre-med," giving us 15 150 pre-medical students on SDN. That brings us down to 10% of pre-medical students using SDN. I'd put the actual number at somewhere between 5 and 15 percent.
Obviously, there are many other factors, some which would decrease my calculated percentage (not everyone is an active member, many people who are "pre-med" at some point don't apply to medical school, pre-meds tend to be more neurotic (i.e., more active posters) than people who have already gotten in to med schools) and some that would increase the percentage (SDN users are more active in their junior and senior years, as time goes on, many people who used SDN for pre-med will use it when they are in med school/residency).
I think the interesting study would be to look at the applicant pool as a whole, find the portion of that population that uses SDN, then compare acceptance & matriculation rates for SDN "users" and "non-users."