Google claims between 40-50% of users
Some obviously use more than one Search Engine.
If you are doing PFP with these guys, get tracking, see what terms are productive, and keep both campaigns running to get the best results (if tracking shows this).
Dont just judge PFP on market share... it is just #*$! extension talk and of little relevance to tracked CPC campaigns.
This may be information that just isn't available?
P.S. Love this forum.