Air France selects what is the most fitting aircraft for their needs (network, economics, performance, customers, ground operations, maintenance, ...)
Therefore they have a large fleet of Airbus A340s (second largest worldwide after Lufthansa, if i'm not mistaken) and Boeing 777s.
This strategy has been successful, as it gives Air France some flexibility on the network for summer/winter season, and switch A340/777 whenever it's needed. Right capacities to the right route.
Air France's CEO Mr. Spinetta has stated the Boeing 777 has much pleased them because of its performances, reliability, economics, but also to the customers, pilots, flight attendants and in a general manner everyone who has to work on the aircraft.