I think this is the most asked question already, and there has been lots of answers. Some are;
Create strong social profiles and pages. The starting point for growing your audience on any social network is strong profiles and pages for your social accounts. This requires including branded cover photos and profile photos. It also requires filling out all of your details.
Share quality content— Even before your startup gets going fully, start building your audience on most networks by sharing content your target audience will enjoy.
Use friend-finding features-Most networks have a way to connect you with people you know.
Add links to your top social accounts on your website. You can do this by adding small square social network icons to your website’s header, footer and sidebar. You can also add the official buttons for each network on your website. You can find the official brand assets, buttons, badges and widgets on Facebook(brand assets and button generator) Twitter, Instagram and LinkedIn.
Add links to your top social accounts to your email signatures. Make sure that everyone you communicate with via email knows about your startup, by adding links to your top social accounts in your email signature. You can do this using small square social network icons or standard text links. You can also use services like WiseStamp to create professional signatures with your social links.
Cross-promote your social accounts. Already have a large audience on one account? Ask its members to connect with you on another. That will be as simple as sending a tweet to your Twitter followers asking if they are on Pinterest, and if they are, they should follow you there, too.
Follow your target Twitter audience and engage with them. Twitter allows you to follow up to 5,000 people. You can follow more after people start to follow you. When you follow people on Twitter, they get a notification that they are being followed. In addition to following people, start chatting with them.