Hi Karl and Jan,
Social media presence is incredibly important in this day and age. For most businesses, it's non-negotiable.
Firstly, a solid social media presence often directly corresponds to credibility. A nameless, brandless store cannot compete with one that has firm digital roots through establishing and maintaining a strong online presence. It's a question of our natural instincts as well -- are we more likely to trust a store with no information associated with it or one you could easily interact with? Most would go with the latter.
Second, there's the obvious marketing pro that comes with having a popular social media account. The more product ambassadors, the bigger the scope of your marketing.
Hope this helps!