These similar-sounding phrases often are used interchangably. Doing so is incorrect. Here is a quick explanation:
Alan R. Andreasen (Georgetown University) defines social marketing is very clear terms:
Social Marketing is the adaptation of commercial marketing technologies to programs designed to influence the voluntary behavior of target audiences in order to improve their personal welfare and that of the society of which they are a part.
Social Media are channels online (Facebook, Twitter, Pinterest, etc.) and/or apps on digital devices.
Of course, Social Media can be used for the specific purposes of Social Marketing.