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I estimate this because of the pattern of clicks we have seen as the scope of the 10 pack widens (it has been covering more places and categories over time, and we have identified new searches by the lowering of visitors for those terms in comparison to overall traffic and with the same organic positioning).
Content providers can count how many click through to them. Most of the useful information is available on the "more info" page without needing to click through to the data provider. Therefore any guesses made by the content provider would be unreliable.
It would be a good project for the eye-tracking consultants.