I understand the math.  Every business since the world's first has prioritized customer support relative to the value of the customer; this is hardly a secret exclusive to PayPal.  Dave's proposition that other sites may alleviate some of these costs seen by PayPal is interesting, but unproven.<p>A more likely case than relying on "maybe users will remember their passwords better" is that Facebook could capture a larger part of transactions (say 30% as App Store does) which enables them to better cover support and fraud.