Can private messages on Facebook be used against me in a divorce/custody hearing?

Your message does not provide any evidence to suggest that the messages, if relevant, would be inadmissible. The closest statute on point appears to be Penal Code Section 632, but I see no facts to suggest that your husband's obtaining of messages fits within that statute. You may wish to raise this issue with the judge, but I doubt that it will lead to the evidence being removed from the record.

Its a rapidly developing area of the law. You can and should challenge it with the court, but it'll likley be admissible.