Comscore Media Metrix has issued a report showing which properties are most popular and which are most effective at getting people to visit often. They're not the same. The biggest sites in February were the usual suspects ... Yahoo (YHOO), Time Warner (TWX), Google (GOOG), Microsoft (MSFT), and EBay (EBAY).
The sites which bring people back most often are those which offer useful services, not just information. Facebook, a site that helps friends and business people connect, is the 36th most visited site but is 2nd most successful at getting people to check back frequently. Weatherbug, with lots of weather forecasts and stats, is number 48th most popular site, but people check it often making it the fifth most successful site for return visits. Here's the Comscore report.
Disclaimer: I own shares of Google, EBay, and Time Warner.