Are Facebook messages admissible as evidence in court?

The short answer is yes, if it is relevant to proving your case.

Yes - if they're properly introduced.

If you follow the rules of evidence, you should be able to get them before the court.

Interesting question - I agree with the lawyers that have posted previously. Aside from email, any Facebook/social media postings that your wife has made may (may, not necessarily will) be admitted as evidence. The practice of admitting social media into evidence is evolving at the moment. Check out the following article regarding the admissibility of social media and sanctions that can be levied: http://www.ediscoverylawalert.com/2012/02/articles/legal-decisions-court-rules/lester-v-allied-part-2-clean-up-of-compromising-social-media-evidence-can-result-in-severe-sanctions/