This topic has been a matter of deliberation for a while now, no wonder FIFA has started to put up a list of fastest footballers with accurate measurement of their speed in km.
Now that Usain Bolts is a professional football player, i think I'll pick him as the fastest. Lol. We all know that isn't how it's been done. Running around the pitch and running with the ball are two different things. Those who are able to run fast with the ball on them are considered as the fastest since it's all about playing ball.
Going back to the question, FIFA has put out a list of fastest footballers with Ajarn Robben of Bayern is number one. His followed by Shane Long of Southhampton, and Antonio Valencia of Manchester United who comes in third place.