Those digital footprints of your web usage make their way back to your FB account, deleted or not. So you are more than watched.

This computer has been on Facebook, and I'm fairly certain that they don't have access to the rest of my web usage.

Then again, I run Linux, so maybe Windows users have some sort of Facebook-malware that pings back to the mothership or something... though we really don't need to look that far to realize that Facebook privacy policy is a problem, do we?

How exactly is one expected to delete all wall postings as well?

They're not; that's the whole point: make the system so hard to leave that no one really ends up leaving.