The market share growth enjoyed by American Express and Diners Club over the past decade appears to have peaked.Three-party schemes, Amex and Diners, increased their share of credit and charge card purchases from around 14 per cent a decade ago to a high of 20.5 per cent in June last year. Since then, their share has fallen steadily, and the latest Reserve Bank figures, released yesterday, show their share was down to 18.7 per cent in January.Amex and, to a lesser extent, Diners were able to increase share by offering high reward points, while Visa and MasterCard had to adjust their offering to comply with the Reserve Bank's interchange reforms. The impact of that change appears to be wearing off.Overall, the schemes are fighting over a smaller pie. RBA figures show that the value of credit and charge card purchases fell 11 per cent in January, month-on-month, and total card balances fell 2.4 per cent.The number of card accounts has grown over the past 12 months - up 1.4 per cent to 15.2 million - but card-holders are spending less. CommSec chief economist Craig James said the average card balance in January was $3197.30 - down 2.2 per cent over 12 months.The principal of MWE Consulting, Mike Ebstein, said balances were at their lowest level since September 2011.Ebstein said debit moved to 40.9 per cent of all card purchases in January - up from 39.1 per cent in January last year.