Fairness requires providing the opposition with copies of court papers, called 'service'.
Recently, Judge Power gave one of our clients permission to serve her husband divorce papers via Facebook. This is one way the courts are adapting to the use of modern technology. These days many businesses only exist in the cloud or people do not have permanent or known addresses and therefore alternative service is necessary.
Normal service in Michigan is by certified mail with return receipt requested. If the defendant does not pick up the certified mail or sign the return receipt, a process server can be hired to serve the documents personally, after tracking down the defendant. If this is not possible, service by publication (if allowed by the judge) is an option, but this is a rather antiquated solution since there are so many publications and many people no longer read newspapers.
So while serving divorce papers via Facebook (private messaging) may not be "traditional", the world has changed and the law is having to adapt to ensure that defendants are notified of lawsuits. In the future, who knows - people may be served via Instagram, Twitter, or even by robot!