On Friday, shares of American Express (AXP.US) fell more than 6% to $318.88.

The company's second-quarter earnings report for the period ending June 30 showed total revenue (net of interest expense) reached $19.637 billion, a 10% year-over-year increase, slightly below the market consensus of $19.69 billion. Net income came in at $3.11 billion, up 8% from $2.885 billion in the same period last year. Diluted earnings per share (EPS) stood at $4.53, an 11% rise and above analyst estimates of $4.40.

However, total operating expenses rose 12% year-over-year, outpacing the 10% revenue growth. This increase was driven by higher customer engagement costs from elevated cardholder spending, the renewal and upgrade of the U.S. Platinum Card, and a higher utilization rate of cardmember benefits. This has sparked market concerns about margin pressure.

Zacks analysts had previously warned that rising customer engagement costs, linked to increased card spending and higher usage of travel and lifestyle benefits, could compress profit margins.