One of the textbook examples of a network effect is the ATM network. The more ATMs in your network the more value your customers will perceive in opening a bank account with your bank. All else being equal customers will pick a bank with a larger network of free ATMs. For very small banks, those with only a hand full of offices, this became a serious competitive problem about a decade ago. To solve the problem these banks formed branded ATM networks. In my region, for example, a large number of very small banks formed a network known as SUM. The SUM network allows me to bank with a small bank but get the advantage of a large free ATM network.

So yesterday I stopped to get money and the ATM machine announced that the bank I was in had withdrawn from the SUM network. My first reaction to the bank