If you want someone to do the Social - then it's a Social Media Manager. If you want someone to focus on advertising than it's a Digital Planner. Go to LinkedIn and type in these titles as if you were looking for a job yourself and see what the jobs and job descriptions are to help you design what you are looking for. LinkedIn is what most digital media people use in our industry.
However, because you are small/niche, you could also offer a competition to a local college for their digital class to come up with a "plan" for you and offer a cash prize (say $500) and the internship to execute the plan for the summer. This would be a huge hit. Make it totally legit and you will get quality applicants. Make if half baked and you will pizza delivery guy who wants to "do social"