ComScore on Tuesday announced the results of a three month study on the smartphone market in the United States. The research firm found Google’s Android operating system has continued to grow, increasing its market share from 47.3% in December to 51% in March. Apple’s iOS accounted for 30.7% of all smartphone subscribers in the U.S., an increase from 29.6% in December. Research In Motion ranked third with a 12.3% share, followed by Microsoft’s Windows Phone at 3.9% and Symbian with a 1.4% share. The study also revealed that more than 106 million Americans own a smartphone, up 9% from December, while 234 million Americans age 13 and older used a mobile device during the quarter. Samsung ranked as the top mobile device vendor with 26% of U.S. mobile subscribers, followed by LG with a 19.3% share and Apple with 14%. A second chart from comScore follows below.