Though it was listed at number 9 in May, Second Life is no longer among the Top Ten most played PC games in the US, according to Nielsen, based on data the ratings service gathers from application activity in 180,000+ homes. For June, Second Life ranked at 11, Nielsen's Bradley Raczka tells me, and what's more, its total share of the market dropped significantly: In June, it had just a .539 share. In May it had a .814 share, and in May 2009, a .996 share. This data belies recent claims from Linden Lab that Second Life is growing, though I suppose it's possible this chart reflects greater market usage of PC games in general.
Looking at the top 10 chart more broadly, isn't it interesting how old most of these PC games are? World of Warcraft has been around since 2004, and most of the other titles have been around for 5 years or more. (And for that matter, League of Legends (2009) is based on a map from Warcraft III (2003), and Sims 3 (2009) is just the latest in a decade old franchise.) Pretty telling datapoint to show that the market for PC games is aging, and isn't being refreshed by new titles.