In the latest research report from Nielsen, data from May suggests that Apple’s iPhone iOS and Android OS are the only two mobile operating systems that are actually increasing in U.S. market share. Nielsen makes the point that smartphones are now the “majority of new cellphone purchases,” and the iPhone is continuing to see increased adoption in the United States.

Following a May survey of U.S. mobile customers, it has been determined that Android and the iOS platform gained a percentage of smartphone market share, while other competitors like Blackberry actually fell in market share.

“Android continues to be the most popular smartphone operating system, with 38 percent of smartphone consumers owning Android devices. However, while Android also leads among those who recently purchased a new smartphone, it is the Apple iPhone that has shown the most growth in recent months.”

This report from Nielsen follows another report that Android saw its market share peak in March, and that Google’s OS is starting to lose traction in light of rapid adoption of the iOS platform.

Whether you think Android is more “popular” or not, another recent survey dubbed the iPhone as the “top-selling handset” on AT&T and Verizon.

Nielsen notes that Android saw a 2% increase in market share from 36% to 38% since April. iOS gained one percentage point to 27% in the same time period. RIM’s Blackberry slid back 2% from 23% to 21%. All other smartphone OS’s remained stagnant below the 10% mark.

Although Android is still ahead in overall market share, Nielsen states that iOS has consistently seen the most new customers. Numbers jumped from 10% to 17% in the last three months.

Blackberry has seen a downward trend in market share for quite some time now, and things aren’t looking hopefully for the once powerful brand. More and more enterprise customers are finding satisfaction in the iPhone and Android devices.

The iPhone 5 is sure to shake up these numbers when it’s released this fall, and it will be interesting to see how the mobile market share landscape looks a year from now.