Shopkick has accounted for a 150-percent increase in wallet share for customers willing to supply their location and shopping information.

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The takeaway from all this is that mobile shopping only strengthens the competition for share-of-wallet.

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"The point is we can grow share-of-wallet as well as the number of stores, " says co-president Robb.

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But in 2008, it shot up to 17-18% of spending, meaning that spiking wallet share for gasoline had to come at expense of other categories.

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Benjamin Ensor, research director at tech specialists Forrester Research, said we were seeing the emergence of "digital-wallet wars" as telecoms, banking services and technology companies all sought to carve out a share of the nascent market.

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