The , created by CMW, challenges Rice Krispies Squares fans to identify a face they do not recognise among a line-up of 49 of their real friends on the network.
They are timed on how quickly they can complete the challenge and those who wrongly select one of their friends instead of the impostor will receive a three second penalty.
Cash prizes and a monthly supply of Kellogg Rice Krispies Squares will be awarded to the best players during the 10 weeks of the promotion, with a number of "unexpected" prizes also up for grabs.
Sam Blunt, Kellogg consumer promotions and digital controller, said: "A Rice Krispies Square defies logic in every way. It's a Square that isn't a square. So it made sense to have a bit of fun with this core brand property.
"We know our shoppers enjoy using Facebook and our app challenging them to 'find a friend who isn't' is a bit of light-hearted fun which we hope will capture their attention."
Kellogg, which owns other cereal brands including Kellogg's Corn Flakes, All-Bran and Crunchy Nut, is currently seeking a new UK chief executive after Greg Peterson departed early last month.
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