I don't know about MyFitnessPal, but on our devices if you do nothing to close Facebook and just open another app, Facebook becomes a service-only process. That means it's not really doing anything, just listening for instructions. In this state it uses an insignificant amount of power and essentially no CPU cycles until something wakes it up.
Google Wallet becomes Inactive (cached). In this state it is just parked in memory, doing nothing, using no power and no CPU cycles.
This is absolutely proper behavior. Facebook and Wallet automatically minimize their impact on resources without manually closing them. In fact, they go to the exact same state if you do manually close them with the back key.
Android since v1.0. Linux user since 2001.