I can tell you this is also true for historious, Chrome is at 46%, Firefox 27%, Safari 20% and IE 3%. Opera is 1.8%, but that's probably all me :( We can't really justify much effort (it's usually large, too) to accommodate 2% of users. Hell, until very recently, our bookmarklet didn't work with IE (some XSS security settings? We never figured it out), but now it eschews the nice overlay it uses for other browsers an…
edit: also, if the service is still new, you might largely be getting early adopters and not supporting IE could inhibit growth.