Facebook can be a battery killer. Especially if you set the notifications for a shorter time period.
Honestly, battery performance is NOT going to be the same day to day. Here are some things that can have an effect (Positive or negative...):
-Temperature changes. really warm, and really cold temps can shorten the battery run time by throwing off the chemical reaction in the battery.
-Cell signal quality. If you carrier is having problems with a tower that you bounce off of, or are working on (Say upgrading it) one, that can effect the signal quality. So, the phone has to compensate. This can mean putting more power to the radio to boost if the Db's are weak. Or, greater processing power to compensate if the signal is noisy (ec/no). Remember, the number of bars is an aggregate of that. Go into settings, About and look at the wireless status and see what the phone signal looks like.
-An app that should have shut down, but is running in the background and needs a force close.
-An app update through the market that is hung and, again, needs a force close.
-Apps that run in the background updating too often (Like Facebook, Weather Apps, IM programs...). If you've added, updated, or changed settings in any kind of app like that, it can eat your battery.
-Apps like the above that also use the GPS (Like FourSquare, Google Maps, Lattitude, My Tracks....). The GPS can really drain your battery.
As you can see, it can be any number of things. More importantly, it can be a combination of many small things.... In other words, the battery is dying the death of a 1,000 cuts!
EDIT - I'm going to ask the mods to move this one into the "Support and Troubleshooting" sub-forum.