Update, 5/20: This chart and summary is corrected here.
I just stumbled onto this March Nielsen Games report with some extraordinary data: based on audience surveys regarding a hundred non-casual, pre-installed PC games, Second Life is the most played of all, registering average playtimes of 760 minutes a week per user, nearly a hundred more than World of Warcraft, and eighth [see link above]
second in total player popularity only to WoW. (Note this popularity based on what people are measured to play say they, and for how long, not on sales figures.) This data isn't an anamoly, by the way: Last January, Nielsen ranked Second Life third in popularity, above Spore and Sims 2, Notably absent from the March top ten are recent bestselling "gamer games" like Team Fortress II, Warhammer Online, and Left 4 Dead. Against those hardcore hits, against even Gordon Freeman himself, Second Life pwns. Image credit: Nielsen.com.
Update, 11:50am: Writing in Comments, Nielsen's Brad Raczka says this data is not from consumer surveys where they estimate gameplay times, but actual play: "Just wanted to let you know that this data is from actual metered gameplay measurement... It is not survey data. We electronically meter the web and application activity from 180,000+ homes in the U.S." In an email exchange, he adds: "Keep in mind the Nielsen Online panel is not a panel of gamers, rather a panel of U.S. homes all over from which we track web and gaming activity."
Update 2, 1:02PM: Massively's Tateru Nino compiles more comparison data, noting for instance that SL's weekly usage numbers seem to eclipse Facebook (47 minutes/week) and YouTube (84 minutes/week).