Actually they have just as good of a chance dropping in numbers as they do gaining. They gained because it was the best looking game at the time and CoD fatigue in some players. It was also plagued with bugs starting off that pushed people away. Then looking at the raw numbers of online players Call of Duty wins.
Call of Duty will continue to sell more than Battlefield. BF has a chance at a small increase or decrease. Either way dont expect much change.
Later on i expect this thread to be revived to admit one of us are wrong and im betting that is you ;)