If in an interview for say a Social Media worker I was asked that question of "How can we promote a brand on social media?"
I would say, there are many ways in which you can promote, build and grow a brand on Social Media because Social Media provides many ways in which to do so. The most obvious way is with a Facebook Page because that allows you to reach and connect to the people who will use your brands products or services that you can share updates to. You can also start a Facebook Group which works a little differently to a Page but allows you to reach much more people. Also Facebook Events are a good way to get people interested in something that is going to take place in the future, such as a product release which can help build brand awareness quickly.
Likewise Twitter lets you reach and connect to people interested in your brands products/services and follow you and get your tweets.
LinkedIn lets you establish yourself as a business and connect to other businesses in your niche/network that you might want to partner with.
Pinterest has a lot of shoppers and they like a lot of images so if you have lots of great photos of your products you can pin them which can go viral.
Instagram is the other biggest social media site where all the cool kids hang out and make their millions now so you'll definitely want to have an IG.
There are other social media sites that provide their own unique way of doing things when it comes to how you connect or how your fans connect to you and how updates are shared for building brand awareness and potentially finding more customers/clients. Google+ is Google's version of a social networking site but it's not as big and dogged a bit but can still be useful in building brand awareness. Also there is YouTube as well of course, can't forget YouTube, a great resource and one any big brand wouldn't ever not be on in some way or another. Whether it's through a user review, affiliates promoting your products or yourself promoting your own channel.
That's about the biggest social media sites you would use. There's also Reddit and Tumblr, Flickr, VK, Vine, Disqus, Snapchat and Whatsapp that you can take advantage of in some way to increase brand awareness and build a following or for your own lucrative endeavour. Provided your brand and its products are in demand.