There is no best way. There is only the way that makes sense to your business. For me that usually includes an integrated approach using multiple marketing activities to reach your customers where they are. That can be a mix of social media, offline marketing, referral programs and email for example. Let's talk more and see what's on your mind.
It can be. It has to be done right and that too depends on your type of business and brand. They say content is king, however, I believe the word relevant needs to be added. Your content is only as good as it is useful to your customers and clients. Provide timely and relevant content and it becomes shareworthy. That's what makes content useful.
Not necessarily. Social media is a type of digital marketing. Digital marketing encompasses all the digital tools available to a marketer whereas social media marketing utilizes a specific subset of those digital tools.