Every social media conference I have attended, often hear at the sidelines, "But social is such a small part of digital, and digital is a small slice of overall marketing"
Social is a subset of digital when you see it from the "media" lens. Digital is much bigger, it encompasses, Search Engine Optimisation, Search Engine Marketing, Online Advertising and many other things I don't know much about.
However, that view changes when you view how organizations have to transition to the new normal. IMHO, then the view flips. Digital is easier to do (internal or external) than social. When I mean easier I am not discounting the behavior change and other change management issues that are critical or something like a CRM or HRMS rollout.
But social, that's a whole new kind of beast.
You know why? Because more than change in behavior, social needs a change in mindset - whether implementing internal social networks or external social communities. It requires moving to sharing, listening rather than broadcasting. It requires organizations and leaders coming down from their pedestal and interacting with customers and employees on one-on-one human terms. It requires trusting that your customers can give you better ideas than your R&D department. It requires empowering and educating and trusting your employees to resolve customers' queries and sharing stories of the organization. It requires marketers to think from the customer-in perspective than the product or service out perspective. It requires new metrics to think about, new goals to measure.
It requires a cultural, structural and process change - in addition to technology.
That is why social is much much bigger than digital.
On that note see this report by the Altimeter Group on the Maturing of Social Media into Social Business in 2013