Can a message on a Facebook Chat be a legal binding contract?

This particular agreement is not legally binding in most jurisdictions because gambling is illegal, and illegal contracts will not be enforced.  However, if you had agreed via facebook to purchase your friend's car for $300, it would be a legally binding contract (barring contract defenses not presdent in your example, such as fraud, mutual mistake, duress, etc.)  Most contracts do not have to be in writing to be enforceable; there is generally nothing invalid about an oral agreement (although obviously more difficult to prove), so the added proof from a facebook chat makes it even more likely that the contract will be enforced by a Court.