Have you updated all your applications from Market? Check if you are using the latest version of Facebook for Android app. Recently there was a lot of updates going on and one of them screwed the battery life of my Android phone. A lot of people complained similar issues too. Then another update came and everything went back to where it is supposed to be.

If this ain't got anything to do with apps (meaning you got them all updated), then probably the battery is still new. Exercise it for a few rounds and if its not getting better, approach SIS for warranty support.