Are you talking about how much system RAM it uses, which can be seen under System/Apps/Running, or how much Application Storage it's taking up, which can be seen under System/Apps/All? System RAM is where the processes and services reside when the app is active, while Application Storage is where the app itself and associated data are stored. Right now, Facebook is using 55 MB of system RAM while the process and services are running; however, it's using 40.95 MB of Application Storage, along with 32 MB of Cache, which is just temporary.