Year to date the top 10 contributors have combined to account for 37% of the S&P 500 index return (more than double their market cap representation of 17%). The concentration among the top five is even greater, with those firms – AAPL, FB, AMZN, GOOGL, and MSFT – accounting for 28% of the return and 12% of market cap.

Some further perspective, courtesy of the WSJ, which notes that the combined market capitalization of AMZN, MSFT, INTC and GOOG makes up about 8% of the Index's total.

Throwing in Apple and Facebook puts about 13% of the S&P 500's combined market cap into the hands of just six companies. This wasn't always the case. Ten years ago, Apple, Amazon, Google, Microsoft and Intel made up just 5% of the S&P 500's market cap, while Facebook was four years away from becoming a public company.