For Pocket Frogs version 2.1.3 or older, neighbors were pulled from your current Game Center friends. With iOS 10 the ability to add new Game Center friends was removed which limited this feature greatly.
With Pocket Frogs 3.0+, the new friend code system will allow you to add friends from both the iOS and Android versions. Any previous Game Center friends were lost and need to be re-added with their new friend code. You can find and share your own friend code from the top of the Neighbors screen.
Players will need to update to the latest version to continue to interact with their neighbors on version 3.0+.