Growth of Google's Android operating system continues to slow, hovering just above 50 percent U.S. smartphone market share for the third consecutive month, according to new data published by research firm comScore.

Android-based devices represented 50.9 percent of all smartphones nationwide in May 2012, up from 50.8 percent in April but still down from a high of 51 percent in March. Android's status coincides with a recent upswing for Apple's iOS: Five years after the retail launch of the first iPhone, the iOS mobile operating system now powers 31.9 percent of all smartphones in the U.S., up from 31.4 percent in April 2012 and an increase from 30.7 percent in March.

Android and iOS now make up a combined 82.8 percent of the U.S. smartphone market. Research In Motion's BlackBerry continues to crumble, sliding from 11.6 percent market share in April to 11.4 percent in May. After several months in decline, Microsoft's Windows platform held steady month-over-month at 4 percent, with Symbian bringing up the rear at 1.1 percent, down from 1.3 percent in April.

Close to 110 million U.S. mobile subscribers now carry smartphones, up 5 percent since February 2012, comScore reports. Text messaging behaviors remain unchanged since February, with 74.8 percent of U.S. subscribers sending a text to another mobile device. At the same time, 51.1 percent of subscribers used a downloaded app in May, up from 49.5 percent in February, and 49.8 percent accessed the mobile browser, up from 49.2 percent three months earlier. In addition, 36.7 percent accessed a social networking site or blog in May (up from 36.1 percent in February), 33.5 percent played games (up from 32.2 percent) and 27 percent listened to music on their phone (up from 24.8 percent).