Just had a look at my discourse logs and think I see the error- it looks like discourse is throwing an exception when it tries to load the avatar I supply during the sign on because the avatar link http redirects to akamai. Here is the entry from the discourse log:
RuntimeError (redirection forbidden: http://graph.facebook.com/1354638543/picture?type=large -> https://fbcdn-profile-a.akamaihd.net/hprofile-ak-xfa1/v/t1.0-1/1780631_10203215619495358_409132
When I tried to load the image http://graph.facebook.com/1354638543/picture?type=large in a browser I can see it redirects to akamai.
Some background, I have implemented discourse sso for my application, and my application allows users to authenticate via google, facebook etc.
In this case it seems to be only users who use facebook who are having an issue, and their discourse accounts all seem to have empty avatar links, so the default avatar displayed seems to be based on the first letter of their names. It looks like this exception causes the accounts to remain unactivated - if I activate them manually then they can use their accounts.