Can information from Social Media sites be used for UCMJ actions? Absolutely.
Anything posted on Social Media can be used as evidence, subject to verification. Once a Soldier has information posted to Facebook or some other media, it is open information to the public. Even though, in your scenario, his wife posted the comment, it can be inferred that she would know his condition. However, getting her to testify or write a statement that her husband was hung over could be another issue. If the Soldier was given an Article 15 and took it to courts-martial, if would be doubtful if the wife would testify against him and she could not be forced to testify. One way I have used Facebook in the past to track down AWOL Soldiers (one kept updating his status location from his iPhone), another way was when a Soldier denied underage drinking, but he had pictures showing himself at a party with a beer in his hand and other Soldiers from the unit with him. Be careful what you post.