An update on the Nielsen study that revealed Twitter's too-low retention rate: After receiving some feedback and criticism, Nielsen updated the study to include applications and services that feed into Twitter (such as Tweetdeck and Tumblr). The results were unchanged. "The results verified our initial findings: about 60 percent of people on Twitter end up abandoning the service after a month."
http://blog.nielsen.com/nielsenwire/...tter-quitters/