What other apps are you running? I get about a full days worth with fairly heavy sms and bbm use while at work. If I mess around with FB (specifically fb chat) I get more significant loss. Posted via BlackBerryForums.com Mobile
Facebook should not be left on, at best you should access it read what you have and exit by closing the app, if you are letting it run constantly your battery will be drained quickly. As much as people use these devices for social apps, they are not designed for that primary purpose.