I was at Microsoft HQ a while back and they proudly said 4 million people in the UK use don't use Google. I stopped and did a quick bit of maths, thats about 6%! (I couldn't get why they were so proud of it till I looked round the room and realised the day wwas aimed at business owners not IMers)
That said, Bing and Yahoo traffic does convert better. Look at your analytics, if they're anything like the sites I manage your average user from those two sites spends far more time on site then the average Google user, it's just there's barely any of them about!
BUT if you were a business looking to advertise, would you rather advertise on the bigger network where next to noone clicks and most of those that do don't convert, or the network with less traffic which is more likely to convert?
It'll be a long time coming, but this is just the same as AOL / IE in the 90's, noone ever thought they'd be toppled either, but they offered shit service and user experience for so long that they got sucker punched by services which didn't treat the user like a revenue stream.