Prob 75% of premeds have used SDN at some point whether as a member or lurker (hell, pretty much every google search brings you here). I bet that less than 5% of those users would be willing to wear a SDN-related shirt...even if it was free. The site has a stigma among its premed users as being competitive and neurotic. Who would want that scarlet letter? Other users wouldn't care about being "judged", but likely don't want to advertise their "secret weapon" to their competition. It's not hard to fathom that SDN usage makes for an improved application...gunner premeds won't want to share the wealth.
I also bet that a similar percentage of med students know of/use the site. But, again, due to stigma maybe 10-15% would admit using it...let alone wear a free tshirt advertising the fact.
Change the parameters by making the tshirt cost 25 bucks and what you have is a waste of time.
If SDN wants to change its perception it needs to sponsor events at universities/med schools and build their own brand... This will not likely happen. Nevertheless, leaving it to word of mouth and premed imagination gives you what you have today: a community based largely on secrecy and paranoia.