Google's Android platform is continuing to make steady gains in market share, increasing by almost a half from February to May.
From just nine percent in the December-to-February quarter, it's risen to 13 percent, says ComScore - and while this may still be relatively small, it's a continuing trend.
Once again, Apple lost share, falling one percentage point to 24.4 percent - although, given that the quarter covers the run-up to the launch of the iPhone 4, this is hardly surprising.
RIM still dominates in the smartphone market with 41.7 percent of subscribers, while Microsoft just pips Android with 13.2 percent. Palm completes the top five, with 4.8 percent.
The're all increasing sales, though, as the overall market is still growing strongly. ComScore says that 49.1 million Americans now own a smartphone, up 8.1 percent from the previous quarter.
Samsung is the leading handset manufacturer, with 22.4 percent market share.
Like Pew, ComScore sees a big rise in the use of mobile apps in recent months. Two thirds of US mobile subscribers used text messaging in May, up 1.4 percentage points on the previous quarter, while browser use rose by 2.3 percentage points to 31.9 percent.
Three out of ten subscribers used downloaded applications, an increase of 2.1 percentage points from the previous period. There was also a lot more use of social networking sites or blogs, with the number of people accessing these sites increasing 2.6 percentage points to 20.8 percent of mobile subscribers.