We have hired a few that have admitted to smoking marijuana before, but the rest of their background was squeeky clean and they stood out from the other candidates. If we find that there have been any illegal use of harder drugs such as cocaine, crack, heroin, meth, etc. (felonies in and of themselves to possess) we will not consider those candidates.
As for the facebook issue the only time we can get the contents of a closed facebook account is in a criminal case, which requires a search warrant. So in other words, no we can't look into your closed account for the purpose of a back ground investigation. Your friends and family members on the other hand may still have pics, or stupid comments made by you on their accounts. You would be suprised at how many people do not set their account to private, which makes our job at digging up dirt on you very easy.
As a background investigator I don't look for reasons to hire a person, I look for reasons not to hire. I spend the least amount of time during a background investigation speaking to your references. We know they are going to speak highly of you. I spend the majority of time speaking to people you associate with that you didnt list in your application / resume,your previous employers, neighbors, etc..