It depends on the user intent. You can use them for your personal purpose and also for Marketing activities.
Social Media websites like Facebook, Twitter, Google Plus, Tumblr, Affimity, Instagram play a vital role in the Online Marketing activities of your business.
They improve your branding and social presence.
We are in fact building the new age social media platform for like minded people who share common interests. The social platform name is Affimity - Social networks reimagined. Niche social networks where people share your passions.
Source:https://www.quora.com/How-useful-are-social-networking-sites