After my Panda changes Bing /Yahoo increased well over 100%, Goog went down to next to nothing and with third rate keywords and traffic is throttled too apparently. I'm 100% sure it isn't due to metrics we know, and I am no longer willing to believe that some fair 'secret sauce' is being used either. Bing uses click satisfaction metrics too and Bing is way pickier on content than Google, the monetizer Pro.
So, are we to understand that panda has been a success or maybe we should ask what is 'success'? IIRC, an engineer of theirs (his name escapes me but he was a G fellow) claimed users are very satisfied. Looks they are showing their alleged satisfaction in a weird way with about 6% loss since Panda, if my memory is correct.
Bing needs to replace the G contract on Firefox and see if they can make Bing a default on IE (I think it's a choice now) since Google has a lot of market share.
Now the bad news: Google needs to make the same money +20-30% a year on less market share.
Both (or all three) are roughly as bad the others so it really makes no difference. And as far as I know Bing is making tiny increases in market share at mouth-watering cost.
I do note though that the Yahoo share is staying roughly the same which shows that the average user can't distinguish between Yahoo, Bing and Google. They are just sticking with the same old search engine they always have.
That's why it makes sense to spend a lot of money upfront and then give them no reason to leave, in other words provide decent enough results.