As the general rule, all Sith will carry the title of "Darth", but Snoke and Ben never have such a title. Are they not Sith?

It is technically possible for someone simply turns to the dark side and not declare himself / herself as a Sith, but the entire Star Wars plot line is based on the eternal conflict between Jedi and Sith.

It is not really logical for them to not be Sith.