Last week, the data analytics firm comScore released data regarding the number of smartphone subscribers in the US and how it has changed over the past three months. As expected, the Android and iOS platforms continue to dominate the smartphone industry in the US, as they remain the only mobile platforms to gain market share over the past three months.

The BlackBerry platform has lost 2 percentage points in this metric over the past three months. This is in comparison to the Windows platform, which has lost a single percentage point. On the surface, this points to RIM losing more smartphone market share than Microsoft. Although this is true, Microsoft has lost a larger percentage of their market share relative to RIM. While RIM’s market share has decreased by 2.0%/17.2% = 11.6%, Microsoft’s market share has decreased by 1.0%/5.4% = 18.5%. Keep in mind that the data is only relevant to the US market for smartphone subscribers of ages 13 and older.

Considering the US is one of RIM’s weakest markets at the moment, it’s interesting to note that it is also there that they are still besting Microsoft. If this rate of change in smartphone subscribers is to continue, Microsoft’s situation in the US is looking quite a bit worse than RIM’s. However, trends are everything in mobile. Microsoft is planning to release its new Windows 8 operating system designed to be used across the broad spectrum of devices from PCs to smartphones. Not to be outdone, RIM is also planning to release its new BlackBerry 10 operating system later on this year. Both platforms are fiercely competing for that #3 spot in mobile platforms.

The Windows 8 platform should slow down the rate of loss for Microsoft, and perhaps begin growing market share. The BlackBerry platform has much larger subscriber base at the moment, which will serve as a great base upon which to grow the new BlackBerry 10 platform. It is still too early to call, but the trends suggest RIM should stay in the lead for a while and is also in a better position to claim that #3 spot as their own.