What’s the ROI on marketing in Second Life? After attacking the topic from a couple directions (most SL users seem receptive, but so far, are largely unimpressed with existing attempts), we finally have some concrete numbers to work with, at least on the more relevant metric of unique visitors.

So how does anywhere from 6,454 to, well, zero, grab you?

That’s the spread of weekly visitors to real world corporate sites in Second Life, according to SL demographics expert Tateru Nino. Every Monday on my Second Life blog, she reports on the number of Residents to visit these locations; the world’s dynamic map enables her to headcount actual visitors, and with some common sense extrapolation (sampling numerous peak and offpeak use periods), winds up with a fairly accurate estimate of total uniques.

In the three weeks since she began, the highest weekly total belongs to Pontiac, which hosts a kind of virtual autobody island where Residents can customize their cars (pictured), followed by a site for Showtime’s The L Word (4,687), where regular events are held on a recreation of the show’s main locations; IBM has an expansive site which includes an open source coding tutorial lab (4826).

The low end, however, is littered with some of the world’s most prestigious corporations and brands. Despite entering Second Life to much mainstream media fanfare, companies like Sears, Sun Microsystems, Dell, Coca Cola, Reebok, Coldwell Banker, and Calvin Klein have so far failed to attract even 500 weekly visitors each (during Tateru’s headcount, at least)— some of them far less.

For comparison’s sake, there are currently about 400,000 Residents who log into SL on a weekly basis; about a million have logged in over the last month. By far the most popular sites remain grassroots “native” locales; over the last three weeks, for example, the most heavily trafficked place was not a casino or a sex hangout, but “Phat Cat’s Jazzy Blue Lounge”, a PG-rated ballroom for elegantly dressed avatars. With a highest weekly performance of 31,248 unique visitors, it vastly eclipses even the most popular corporate site.

So roughly a year after SL’s mini-dot com boom began, most companies are still struggling to even be noticed. I recently put these paltry figures to Steve Prentice of Gartner, as a caveat to his firm’s bullish predictions that 80% of Fortune 500 companies would have a virtual world presence by 2011. He seemed surprised, then fingered the relative lack of company avatar activity at most of these locations.

“[Y]ou’ve got to be able to go in there and interact [with people],” he said, “that’s the nature of 3D interactivity.” Though many companies will quit in frustration, Prentice believes they’ll learn from their early botched attempts— whether they want to or not. “Like the [early] Web,” he added, “most will be brought back to the table due to user pressure.”