Android is coming, a report released today by web measurement firm comScore Inc. shows. And that means e-commerce companies considering mobile apps, and how to optimize mobile sites, should think beyond the iPhone and BlackBerry.

The comScore research, conducted over the three months ending in May, ranked the top mobile smartphone operating system platforms in the U.S. according to their share of mobile subscribers age 13 and older, and reviewed the most popular activities and content accessed via mobile phone.

RIM, or Research In Motion, which makes the BlackBerry smartphone, led among smartphone platforms with 41.7% market share, followed by Apple Inc. with 24.4% and Microsoft Corp. with 13.2%. However, Google, which developed the Android operating system, experienced strong growth during the period, rising four percentage points to capture 13% of smartphone subscribers for the three-month period ended in May, up from 9% for the three-month period ended in February. Palm rounded out the top five with 4.8% of the market.

Despite losing share to Google’s Android, however, most smartphone platforms continue to gain subscribers as the market overall expands, comScore says. 49.1 million people in the U.S. owned smartphones during the three months ended in May, up 8.1% from the prior period ended in February. In a positive sign for online retalers, 31.9% of U.S. mobile subscribers used mobile web browsers during the period ended in May, up 2.3 percentage points from the prior period, ComScore also reports. That means those consumers can access retailers’ mobile commerce sites. And 30% of mobile users downloaded apps, up 2.1 percentage points from the previous period. In addition, 20.8%  of mobile subscribers used their devices to access social networking sites or blogs, increasing 2.6 percentage points.

The comScore data show that e-commerce companies that optimize or create mobile apps only for the iPhone, BlackBerry, Microsoft and Android operating systems are reaching more than 90% of the smartphone market.