Police, government agencies and prospective employers have been using Facebook and other social media sites to find information about people for some time. But it is actually also legal for documents to be served on people via social media such as Facebook (actually it has been for quite awhile).
In many cases it is easy for people to hide their physical address. But if they are actively using social media for regular posts, you can find them relatively easily if you need to!
For legal advice on this and any conditions, check with your legal advisor.