For optimal syncing, we suggest using iCloud (with Apple devices) or Google Contacts (on Android devices).
Below is a step-by-step guide on how to optimally setup your FullContact account and then sync to your devices:
1. Create a FullContact account here.
2. Connect Google Contacts, iCloud contacts, or your Exchange / Office 365 address book.
3. Connect social networks.
4. Enable iCloud / Google / Exchange as your *only* contact source on your devices. By doing so, you'll alleviate seeing duplicates in the native Contacts app of your smartphone.
On iOS devices:
Settings -> Contacts (iOS 10 and newer) / Mail, Contacts, Calendars (iOS 9 and older) -> Accounts - enable only iCloud access to Contacts.
On Android devices:
Android Device Settings -> Accounts & Sync -> Connect your Google account that is syncing with FullContact.
System Preferences -> Internet Accounts -> connect iCloud, allow access to contacts.
You're all set! FullContact is now syncing across your multiple devices and you have one, unified address book.