You must promote one way or another but social media is only one way to do that. I picked up a good portion of my subscribers promoting my channel through business cards, attending social events, community centers, etc. With that said, social media is the easiest and cheapest way to promote. It depends on what type of brand you have but I have a baking channel so I do a lot of baking demonstrations at churches, charity events, fairs, etc. The followers I pick up during local community events are more dedicated and more likely to tell their friends and family about my channel. It's easier to build rapport with someone you personally meet then someone thousands of miles away. Think outside of the box and try all different ways and don't limit yourself to any one way. I promote via social media sites as well.