It’s no big surprise, but official now, I guess: Texting is now teens’ No. 1 reason for using cellphones, but using apps is their fastest-growing activity, ReadWriteMobile reports, citing Nielsen research. This year 43% of US 13-to-17-year-olds say texting is their top reason for mobile adoption. “Safety, the number one reason back in 2008, has now fallen to second place with only 35% citing this as the top reason,” according to ReadWrite Mobile. American teens now send and receive an average of 3,339 texts a month (teen girls 4,050). But all teens have lots of other uses for phones – 94% are using multiple smartphone features – browsing the Web, photo-sharing, playing games, and using apps. The biggest growth area over last year was “software downloads,” at 12% growth, but all activities showed growth. After texting, the top activities are photo-sharing (which 62% of teens do) and Web browsing (49%). Software-downloading and email tied for 3rd and 4th place at 38%. [Here’s Nielsen’s own report, which seemed to be down when I tried to access it today.