According to Google Ad Planner, which is now my go-to source for tracking web traffic, the SL Marketplace subdomain now gets about 430,000 unique monthly visitors (give or take). As you can see by the screencap above, the usage rate has been fluctuating at around the same levels since January 2011. This is interesting, because back in October 2011 (the last time Linden Lab reported user stats), the company reported that 475,000 SLers were economic participants. (I.E. spending Linden Dollars.) So I think it's likely that most monetized SLers are at least visiting the SL Marketplace, if not shopping there. In any case, 430K is a lot of uniques. (Last we heard, there's about 1 million total monthly uniques in SL, with most of those being extremely casual users.)
As for SL content creators, I think the takeaway for this data is pretty clear:
Your biggest potential market who are mostly to buy your stuff is not in-world, but on the web-based Marketplace. So it's probably a good idea to optimize your offerings there not just for existing customers, but for the hundreds of thousands of folks randomly browsing content online. For that matter, it's probably a better idea to spend more time improving your Marketplace presence, where you're way more likely to attract customers, than your in-world shop, which apart from the really big stores and brands, likely gets much less traffic. It's also another indication that SL as depicted on the web is at least as important as SL the 3D client-based experience itself (if not moreso).