Did your friends try visiting each city to see if it was CityA or CityB or they perhaps both point to this new CityC?
If the CityA is still there then it might be possible to get it back if support does such things.
The problem to me seems to be with the facebook link being replaced. You may try seeing if there is a way to cancel the link via facebook settings though I'm not sure such a setting exists. Try doing a google search. However, there may not be a way to get the data of CityA unless simcity support can help you in some way.
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