The Law Times has a longish story on a Kingston Ontario judge who allowed service on a family law defendant by Facebook. It is not clear in the story how much of an effort the plaintiff had to make before getting permission to serve that way. The judge seems to have given retroactive blessing to the method when it was shown to have worked (because a reply was received).
The judge gave a talk to a conference about how well this had worked. Other commentators discuss why such service would be a good idea, and at least as a method of substitutional service, better than putting a legal notice in a newspaper.