It has been five years since the first message was posted on the micro-blogging site Twitter. To discuss its significance, Newshour brought together Mohamed Yehia from the BBC's Arabic Service and Richard Coope from the Radley Yelder company.
Martin Strel has already smashed four Guinness World Records after swimming the entire length of the Amazon, Yangtze, Mississippi and Danube rivers. Now the 56-year-old is setting his sights on conquering the Grand Canyon's Colorado River.