I'd asked why 85% of people do not return to Second Life in my post, "Why 85% Of First-Time Second Life Registrants Do Not Return,"
"I'd be curious about the average repeat visitation rates (or non-return rates) for other registration-required sites or services. If they are comparable, then the issue might just be that people like to experience surf and that's just life. If Second Life has a number that's way out of whack I'd be interested in knowing why."
Christian Schade who ran three national Yahoo! portals and was the former Consumer Products & Services Manager, Nordic at Yahoo! answered my LinkedIn question,
"I don't know if this is useful, but you can look at it the other way round: If you are the owner of one of the large, complex multi-application/omnibus portals like Yahoo!, MSN or Lycos, you can normally assume that once you have the user registered for 2.7 of your services, you can assume he is there for keeps."