Guys! I just wanted to share a REALLY simple explanation and solution for something similar that happened to me! Your case might not be so easy, but it's worth knowing.
I had two cities connected via Facebook connect that could see each other and trade. Both were on iOS devices. I upgraded my phone. Facebook connect worked great with backing up my city - I could sign in and load it easily. BUT - I couldn't see my other city, even though we were friends in Facebook! Then I found out the other city COULD see me, making it even more mysterious!
The solution:
Turns out the problem wasn't me but them! When you register an App with Facebook connect, it gets a certificate that only lasts a certain time. (I don't know how long exactly, but definitely not more than 6 months.). HOWEVER, this certificate is only CHECKED when a new connection is made. So you can have two SimCity cities trading forever even though both their App certificates have expired. But once you RECONNECT, those with expired certificates won't show up.
In my case, the MIGRATED iPhone city was fine - that's why the original city could see me. But I couldn't see the original city anymore because its certificate had long since expired! So I went into the original Facebook account, and went to account->security->settings, then on the left choose "websites and apps", and there SimCity Buildit was no longer active but expired. So I went to the expired list, clicked on renew, and it went active again!
But it STILL wasn't visible - until I swiped up on SimCIty and relaunched. Then both cities could finally see each other again!
Bottom line: Gotta ask all your friends to go to Facebook and renew their SimCity certificates.
Worked for me.
