The smartphone space has become crowded. Apple, Google's Android platform, Research In Motion's BlackBerry OS, Microsoft's Windows Mobile platform and the Symbian OS are all competing for real estate on new smartphones. ComScore ranked the top five smartphone operating systems in the U.S. by market share. The numbers were derived from a February survey by comScore, and represent current phone ownership at a specific point in time rather than the total number of devices sold with a particular operating system. Slideshow