Parts per million, actually. Bearing in mind that a good few thousand users play, it's the most sensible unit for showing your contribution to the interaction points.
Page 4. I believe the higher your portion per million, the higher your portion of the total points. Dividing it out into decimals normally to get a percentage becomes a very very tiny number with a very long decimal count if your calculator doesn't resort to using -4 indicators and such. So PPM seems to work just fine for this scenario even if people aren't used to it. Leaves an easier number to calculate from.
