First, as previously mentioned, please do not make your decision based on the individual's social media followers. Instead, you might want to look at some of the growth they've helped to engineer in their previous jobs. For example if they tell you that they helped a company grow from 500 to 50,000 twitter followers, that is pretty impressive. However, even then it is important to take those numbers with a grain of salt. Ask how many were purchased vs earned. Ask what specific techniques they used to gain those followers and how they would apply those techniques to your company. Try to determine how much impact that specific person had on the growth.
Second, just like many people suggest assigning a coding project to a candidate to determine if they fit with the company's engineering style, you can assign a project to a marketing candidate to see if the work they do fits with what you're looking for. Ask them to write a sample blog post for you. Ask them to write a sample email they would send to potential customers. Ask them to outline some ideas they have for how to market your company and how they would implement them.
Third, if you want to attract good marketers, please don't treat them like second class citizens. There is a large resentment among many in the tech community towards "Business" people and "Marketers". If you and your company show similar resentments, the person you are interviewing will be turned off.