If it's really bad (this is where the lack of an anon function is a problem, since you can't really tell us as yourself, I presume) and you don't think you want to go out with him anymore, think of another reason and quickly split up with him! You'll never have to tell him what you did.
...which was a really terrible thing btw. Maybe facebook isn't so bad (not as bad as a diary, for example) but I was still pretty annoyed when two of my friends went on mine the other day; they changed loads of things, and just generally strung me along that they'd messaged people. Especially after one of them left their facebook open on my laptop and I logged them off, because I wouldn't invade their privacy by going on it.
If you do tell him, you'll have to be careful how you put it if you do want to continue to have a relationship with him. I never trusted my ex-boyfriend properly again after he read my diary.