A contract, including an oral or verbal one, is enforceable. A mere statement that person A owes person B money is not necessarily a contract, however; it may be a mere statement or unenforceable promise. Only if there was consideration for the debt--i.e. that your ex-boyfriend received something for it--might a contract be formed an enforceable. For example, if you loaned him $2,000 (as seems to be the case) in exchange for his promise to repay, that is enforceable. In that case, the Facebook statement could certainly be evidence of the agreement, which you could use to prove that it existed and its terms, if you sued your ex-boyfriend and took him too court.