New numbers from comScore Networks show Microsoft’s mobile market share falling to 3.9 percent of total smartphone subscribers in the U.S. in the three months ending in March — another significant decline at a time when Microsoft is hoping for a rebound instead.

But here’s the big catch: The numbers don’t reflect the U.S. launch of the Nokia Lumia 900, the new flagship Windows Phone, which hit the market in early April.

Google saw its Android market share rise by 3.7 percentage points over the same three-month period, to 51 percent, while Apple rose 1.1 percentage points to 30.7 percent. RIM’s BlackBerry market share fell 3.7 points to 12.3 percent.

Here’s the chart from comScore, and see this news release for more details.

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