Facebook user base grows up to 900 million people around the world. There are more chances to find a family member on Facebook than any other social network. Facebook Family Search feature search people with the same Last Name or surname.
Surname indicates family, cast, religion, and some times geo-location of the person. If you are living very far away from your family or home town for a long time than a Family search can bring you closer with them. Sometimes people from a same family, village or geo-location uses same & unique last name.
To do Family Search on Facebook, type your last name (or surname) in search field and hit Enter key. You will find many people of same surname. If you want to broaden your search and need exact match then try Facebook Family Search. This way you can search all the people on Facebook with same surname.
This feature is more useful if your surname is very unique and only few people uses it. I suggest you to give at least a single try, it would be a fun.