The pie chart at right, published Friday by the mobile analytics firm Flurry, illustrates just how rapidly the platforms that brought Mario to a generation of videogamers are shrinking.

It shows Nintendo’s share of the multibillion dollar U.S. portable software game business collapsing in the space of a year from 70% to 57% while Apple’s (AAPL) iOS and Google’s (GOOG) Android’s share grew from 19% to 34%.

Sony’s (SNE) share is down to 9% from 11% in 2009.

Driving the change are waves of free or inexpensive programs designed for casual smartphone use. Apple’s App Store lists nearly 100,000 games and entertainment programs for the iPhone alone, according to 148apps.biz.

“The net effect,” writes Flurry’s Peter Farago, “is that the U.S. portable gaming category, as we define it, has declined from $2.7 billion in 2009 to roughly $2.4 billion in 2010.”

Below: Flurry’s pie charts for the broader video game market, including consoles, portables and iOS/Android devices.

Also on Fortune.com:

• The PC market as a giraffe
• Bloggers vs. Pros: A $2 billion gap
• The triumph of ‘good enough’ computing

[Follow Philip Elmer-DeWitt on Twitter @philiped]