Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
You can't but you can sign out of facebook so that someone can't use your account. You have the ability to put a passcode lock on the iPad so you can lock out all access
 
Put a passcode to access device, now all essentially have a passcode.

You might find something in the jailbreak community, but nothing legitimate.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.