Microsoft Internet Explorer 8 is losing market share with it hitting a historic low since 1999. Currently it rests at just below 60% at 59.95%. About 2.5 years ago IE had a market share of about 80%, so it gradually losing each month as the graph below shows.
From what we see in reports, Google Chrome is the browser that seems to be picking up most IE users as this particular browser has been growing in usage.
Mozilla Firefox claims about 25% of the market and Chrome at 6.7%.
It isn't really clear why IE is loosing so many users other than general problems and vulnerabilities that get reported that could be playing a part in people wanting to switch.
The graph below shows various browsers and their progression over the last 15 or so months.