Amazon Not Friendly to PayPal

That shrewd approach to building momentum is a huge advantage for Amazon over Google. Amazon realized that users won't sign up unless many Web sites take payments from that service, and that many Web sites won't bother to set up a service that doesn't come with a large group of users.

"Any new entrant into the payment processing space faces a classic 'chicken and egg' problem," Amazon senior manager Jeff Barr wrote on the company's blog. "Fortunately, we have both ends of poultry life cycle covered."

While it's much preferred by users than is Google's Checkout, PayPal receives many complaints, as well. In a June interview with Bloomberg, CEO Meg Whitman trumpeted PayPal's triumph by claiming that an internal survey showed that only 14% of Checkout users were satisfied with the service.

A survey in January by JPMorgan found that 19% of Google Checkout users said their experiences with the service was good or better; 44% of PayPal users said the same about their experiences with PayPal.

In other words, more than half of PayPal users may stand ready to give Amazon a shot.