Internet Explorer lost the largest amount of usage share last month in three years, a web metrics firm revealed.

The slide of Microsoft's browser, which dropped 1.8% to 52.6% during October, came on the heels of a fall of nine-tenths of a point in September . Only IE's November 2008 plummet of more than 2% was larger, according to data from Net Applications.

In the last three months, IE has lost 3.3%, or 6% of its total share as of July 31, the biggest three-month drop since October-December 2009.

As has now become rote, Google's Chrome and Apple's Safari were the winners in the browser race in October, boosting their shares by 1.4% and 0.4% respectively.

Chrome ended October with a 17.6% share, while Safari accounted for 5.4% of all browsers used globally during the month. Both were records in Net Applications' tracking of desktop browser usage share.

IE9 increase

Microsoft did not directly address the continued decline of IE, but instead stuck to the message that it has used for much of the year, that Internet Explorer 9 (IE9) has increased its share on Windows 7, the company's newest operating system.

Worldwide, IE9 had a share of 22.5% on Windows 7, an increase of 1.4%. That put the browser in second on Windows 7 behind only Microsoft's own IE8, and far above the third-place Chrome 14, which accounted for 18.1% of all browsers used on the operating system.

In the US, IE9 had an even larger share of 34.9% on Windows 7, more than that of all versions of Chrome and Mozilla's Firefox combined.

Older IE users defecting

While IE9's gains have been impressive, the new browser - launched last March - has not stopped defections from older versions of IE to alternate browsers.

In October, IE9's share of browsers running on all operating systems, not only Windows 7, grew by 1.1% to 9.8%. At the same time, however, IE8, IE7 and IE6 lost a collective 2.7%, or more than double IE9's increase.

IE8's share dropped 0.0% of a point to 29%, while IE7 and IE6 lost 0.6% and 1.1%, respectively, to end October at 5.4% and 7.5%.

It's possible, of course, that when older versions of IE near extinction, that the desertions will slow or cease. But by the time Microsoft's long-game plan plays out, IE will have lost its majority position and fallen under the 50% mark.

Chrome on Firefox heels

According to projections based on Net Applications' data, IE will slip under 50% as early as January 2012, and if the losses of the last three months continue on their torrid pace, Microsoft's browser will account for just 43.7% by June 2012.

Chrome gained most of the share that IE lost, continuing a trend established in late 2009 when Firefox's growth stalled. Firefox ended October with a 22.5% share, unchanged from September.

Firefox remains in in danger of losing its second-place spot to Chrome: If the two browsers keep to their current trend lines, Chrome will overtake Firefox in April 2012.

Chrome should crack the 20% mark in either January or February 2012.

Net Applications calculates browser usage share with data obtained from more than 160 million unique visitors who browse 40,000 websites that the company monitors for clients.