According to web usage data from the company, at the end of April, IE had dropped 0.7 percent to a 66.1 percent share of the browser market.
While IE8 boosted its share by 2.2 percentage points, IE7 lost 2 points and the creaking IE6 lost 0.8 percent point.
"Is there an end to IE's decline?" asked Vince Vizzaccaro, executive vice president of marketing for Net Applications. "I don't know. I never thought they would drop this far."
Although IE8's gains originally came almost entirely at the expense of IE7, Net Applications' April data shows that IE6 users are also starting to upgrade: The older browser's 0.8 percentage point loss last month was higher than the 0.6 point drop the month before.
Microsoft recently started offering IE8 to IE6 and IE7 users via Automatic Updates, a factor that may have played a part in the accelerated decline of the older browsers and the uptick in IE8.
As usual, rival browsers picked up IE's losses. Mozilla's Firefox, for example, increased its share by 0.4 of a percentage point to end the month with 22.5 percent, while Google's Chrome climbed 0.2 of a percentage point to 1.4 percent.
Apple's Safari, however, missed out on the action, and slipped 0.02 of a percentage point to 8.2 percent.
"IE had such a huge market advantage, and Firefox, in a competitive environment, continues to gain share," said Vizzaccaro. "We've seen some seasonal flux [to Firefox's share], but now it's just continuing to go on an upward trend."
Unless Microsoft is able to increase users, IE will lose its majority status and fall under 50 percent sometime in May 2011, Net Applications predicts.
Google's Chrome also caught Vizzaccaro's eye. The browser, which remains a Windows-only application, had its biggest increase since last December.
"We've seen Google putting ads [for Chrome] on its search page, and this play on marketing has given it a nice little bump," said Vizzaccaro. "But I don't know if that kind of increase is sustainable."