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I think this is mostly to do with cookies not working cross domains (for obvious reasons).

• jessaustin 14y ago
  This seems likely. Still, with a bit of planning they could have gotten around this. Everything dealing with identity could have been served from a single domain. There probably would still have been issues, but they would have been resolvable.
• TazeTSchnitzel 14y ago
  But Google manages to do this, I'm unconvinced.