The U.S. added 31 million smartphone subscribers in the past four quarters. That represents a reacceleration of growth in U.S. smartphone penetration.

For comparison, the U.S. smartphone market had added 28 million subscribers in the four quarters leading up to the end 2012.

However, this quarter’s 12-month growth rate is still below the rate reached in the third quarter of 2012. At that point in time, the U.S. had added 32 million subscribers in the preceding 12 months.

In fact, growth peaked in the fourth quarter of 2011, when 35 million more Americans had a smartphone compared to a year prior.

Overall, U.S. smartphone penetration stands at 58% of mobile subscribers aged 13 and above. That’s up from 54% at the end of 2012.

As we’ve discussed before, penetration growth slows after adoption hits 50 percent because remaining market segments tend to be older, less wealthy, or late adopters. While growth was up slightly this quarter, we expect it will continue to taper down going forward.