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If you don't know them, you are most certainly over 20. Sweden's PewDiePie (on the left) has almost 40 million subscribers on YouTube. Jenna Marbles (in the middle) makes an estimated 350 000 $ annually via her channel. And Thatcher Joe (on the right) has 3 million subscribers on Instagram and has just published a graphic novel about what his fans online have shared with him. Our correspondent Vincent Dowd looks into how attractive these YouTube Stars are for big businesses and why. (Pictures: From left to right - PewDiePie, Jenna Marbles, Thatcher Joe; Credit: Getty Images)