One of the most important things you can do to build your social media presence is incredibly simple. One word: RESPOND.
• When people message you on Facebook, respond. Quickly.
• When people retweet your content on Twitter, respond.
• When people comment on an Instagram post, respond.
In order to do this you need to make sure all of your social media sites are setup to notify you when someone engages with you in some way. If you have a large following and receive a lot of comments, messages, retweets, etc. then it would also make sense to use a social media monitoring tool like Hootsuite, Buffer or Mention.
It may be a simple thank you or it may generate a great conversation with a fan/follower. Either way, it will show your audience you are listening and are interested in what they have to say.