Soon after the iPad was released, I asked whether nonprofit groups should start to develop ideas for engaging with supporters on tablet devices such as the iPad.

The question seemed premature to some, since the newly minted Apple device was being used largely by early adopters and because its price seemed steep for many nonprofit organizations.

But as the devices have been catching on, it’s becoming increasingly clear that tablets will soon become as much a part of the mainstream as netbooks and desktop computers.

In fact, a report by Forrester Research predicts that tablets will outsell netbook computers by 2012 and will account for nearly a quarter of all computer sales by 2015.

“By 2015, Forrester forecasts the US PC market will break down as such (based on percentage of units sold in 2015): notebooks (42 percent), tablets (23 percent), desktops (18 percent), and netbooks (17 percent),” the company said in a release.

If the prediction is true, tens of millions of people will be using these devices in the next five years — and some of them will be your organization’s most enthusiastic donors and volunteers.