Social media the best way to gain traffic is to run a series of promotion through social media. Best website to do this is on Facebook; make sure you will involve your website to your promotions, games, challenges and etc. to make sure that you can gain a lot of traffic.
facebook is most popular social site. but if you have a business than i think only cover facebook isn't enough here are some sites
fb, twitter, linkedin, instagram, snapchat, tumblr, google+, you tube, pinterest, flickr so on.