According to the graph provided by the Niesen website, which we explored earlier in terms of Wii usuage, the Xbox 360 actually came out on top of the PS2 in terms of overall percentage of minutes used, the first time a current-generation console has managed to top Sony's last-gen console. It went on to lose the lead in June, perhaps due to one particularly active Nielsen family going on vacation or something. That's the thing with the video game Nielsens...it's relatively easy for one user to make a difference. Then again, it could be indicative of a larger trend. Either way, Microsoft has issued a celebratory press release, so way to go 360!
Microsoft also points out that the Xbox 360 was also tops in terms of active user percentage, maintaining 11-12% of the active users from January through June of this year, as seen in the graph below.