Where can I find runway usage statistics of USA airports?
Like @CrossRoads said in his answer, there is no data available on which runways are used most. You will need to listen to LiveATC on several days and keep track of the winds to get a rough estimate. For example, from listening to the tower at KSFO I gather that they usually land on 1L and 1R and take off on 10L and 10R. However, when the winds are too strong out of the east they use 10L and 10R for landing. 

Likewise, if you listen to the tower at KJFK you can tell which runways are in use on any given day. There appears to be more variability there because the winds are much less consistent.

And then there are airports like KOAK and KSJC that have one runway or set of runways that GA aircraft use and one for the big guys.

The airports might keep that kind of information so a call to a few of them might give you some answers, but I am not aware of it being published anywhere.