It depends on your audience, and what kind of product/service you're providing for your target demographic. There are many social media platforms out there with millions of users daily, such as Twitter, Facebook, Instagram, and Tumblr. If you know your target demographic, you can use certain hashtags to guarantee that people will see your advertisements and posts. Instagram and Tumblr are some places where it's really easy to build a large following - Instagram especially. If you know how to utilize your hashtags, you can start racking up followers - just be sure to post systematically, using apps like Later and Storrito to schedule your posts and be prompt. Spamming your audience with many ads per day, followed by a week of radio silence is a good way to get unfollowed. You should make your audience want to see more content from you.