5. Instagram

Instagram launched in October 2010, and 25,000 people signed up on the first day. By 2011, Instagram was Apple's iPhone app of the year, and by April 2012, Facebook CEO Mark Zuckerberg shelled out $1 billion to buy the company.

Instagram now has over a billion active users and has branched into live video, stories, and face filters, and has added a long-form video service called IGTV.