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Short answer: having one login is indeed a different question than having one service that can do everything.
The one login do exist, it's called " Open ID ", and the majors IT companies offer possibilities for doing that. For example you can login into 500px, AirBNB, CouchSurfing, etc with your Facebook or Google profile.
The one service that can do everything would be possible only if every new feature, that a new company invente and create, would be immediately published into a "Open Something" norm that describe it and the way to interact.
