Digital marketing is often accomplished by an individual with a computer (mobile, tablet, desktop, or otherwise), the Internet, and some smarts. As digital marketing matures and search algorithms improve, the worlds of SEO and links meld with Public Relations, design, and more.
No longer can a modern digital marketing effort be executed with one individual, no matter how intelligent she may be. I am a firm believer in the full stack marketer, but when there are dozens of web assets and strategies, it no longer becomes an issue of skill sets in the individual or the ability to handle the complexities of digital strategy, it is more about the sheer volume of work that deserves the time needed to produce quality.
Digital marketing now requires building a brand. A single mind should drive it, but it will be impossible for them to execute it alone.
Good thing we live in a world of smart freelancers, contractors, and communities.