I think Parse user management is aimed at a different use case. The AccountManager is a more heavyweight library that is useful for many complicated things. For example, if you are Facebook or Foursquare and you're implementing your own api for third parties to use and you want third party apps to be able to use your service's account, definitely use the AccountManager. Whereas the ParseUser is designed to be the simplest way to add your own user accounts to your application. They're both useful; use the right tool for the job.
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I think Parse user management is aimed at a different use case. The AccountManager is a more heavyweight library that is useful for many complicated things. For example, if you are Facebook or Foursquare and you're implementing your own api for third parties to use and you want third party apps to be able to use your service's account, definitely use the AccountManager. Whereas the ParseUser is designed to be the simplest way to add your own user accounts to your application. They're both useful; use the right tool for the job.