Second Life has maintained its position on the latest Top 10 most played PC Game chart from Nielsen ratings, based on data the service gathers from application activity in 180,000+ US homes. This latest Top 10 chart is from April 2012 (at left, click to embiggenate), with a .867 audience share and 1.543% of the total minutes played (or TMP) on PC games for that month*. That's an increase from the March 2012 ratings, when SL had a .634 share and 1.263% of TMP.
However, while share has grown, average minutes played per week has dropped:
For SL, it was 499 per week in the March 2012 lists, and has sunk to 401 minutes on the April chart. (I.E. from about 8.5 hours a week, to just under 7 hours a week.) However, other top MMOs also saw a drop in average play time -- WoW, for example, went from 534 minutes average in March to 466; LOTRO, from 477 to 348. Comparing March and April, there's a general drop in play time for most games on the list, so my guess is this reflects a seasonal change, as the real world (at least in the West) gets warmer, and outdoor fun becomes a more viable alternative.
*Pre-emptive reply to Dwight Scrhute-esque "But Second Life is NOT a game" complaint: Yes, Second Life is not a traditional game by some definitions, but then, as a PC-based client that's primarily used for 3D graphics-driven, avatar-based entertainment and game-like interactivity, from Nielsen's perspective this is the only appropriate category to put SL in.