Kris Sanchez joined Twitter in 2009, primarily to follow Britney Spears! Short of anything interesting to tweet himself, he started tweeting interesting facts he’d found online. This mushroomed into Uber Facts, a business that now makes half a million dollars a year by building galleries and tweeting links. The operation now has a presence across Twitter (9.4 million followers) Facebook (1.27 million likes)  and Instagram (468,000 followers).

Not everyone can (or will) make a fortune on Twitter, but it costs absolutely nothing to try. What other business could you start from your mobile phone for nothing and turn it into half a million dollars a year within 5 years?