It's very-very depended on your target audience. Try different channels and use analytics tools in the first time. After several weeks or months you will be able to determine what marketing channels bring the best traffic. I personally use Google Analytics. They also allow you to track users' activity on the website and track the conversions with overall user behavior (page where the session started, source, medium etc.)
Use analytic tools to research your audience and find answer. From our company's experienceand experience of other companies I worked with, I can only say that we always used several channels. For some products Facebook and twitter worked best, for others - LinkedIn, but you will know for sure, what's the right fit for you only after conducting a careful research.
By the way, in this article
you may find a review and list of marketing chanels for an app, if you're interested in how app developers manage their marketing campaigns.