Keep this in mind: Their understanding of psychology, technology and communication is much more important than how many followers they have on social networks. Someone's social network activity is often misinterpreted as being important in helping grow a company. That is really not the case in most circumstances. Try to find someone who is up to speed on the latest tools, but maintains a healthy level of skepticism to them. They need the ability to help your potential customers become aware of and understand your product. There is a flip side to this as well. A good marketer will be much more than just a mouthpiece for your company. They should be able to communicate the needs of the market back to you as well.