This is how I fixed my phone... I disclaim any and all responsibility for anyone following these instructions!!!
I *FINALLY* found the culprit in my Verizon HTC Droid Incredible phone with Android 2.3.4 Gingerbread.
It used to be, that Facebook or the Contacts Directory, or sometimes Mail were the programs taking up space.
But it turns out on my phone at least, I found there is an error log that keeps growing, and it's gotten worse after the Gingerbread update.
To deal with this, you have to root your phone. I've had this phone since it came out, and I've never needed to root until now...
Instructions are at 2.3.4 root,downgrade,and s-off - Android Forums
NOTE: You do NOT have to downgrade and you don't need "S-off", you only need to follow the directions up to where it says "you should now have the superuser app in your app drawer." Be sure to read the special note to users with the SLCD screen.
Go get "Root Explorer" from Market, then navigate to the following directory:
And there is a file called anr_history.txt
For me, it seems to grow by more than 300K per day with unneeded crash log data. Mine was 70+ MB!!!
Delete the file, it's not needed for anything.
This site: HTC Desire Custom A2SD Sense ROM: Free space dwindling gives instructions on how to prevent it from regenerating... when you reboot your phone, the file is re-created. You have to remove write permissions and then change the ownership of the file so that it will not grow again. This can be accomplished with Root Explorer.