Listed first are instructions for syncing via iCloud / Google / Exchange:
1. Create a FullContact account
2. Connect Google Contacts, iCloud, or Exchange contacts
3. Connect social networks
4. Enable iCloud / Google / Exchange as your only contact source on your devices.
*Multiple accounts set as sources may display duplicates even if they are identical.
On iOS devices:
Settings -> Mail, Contacts, Calendars -> Accounts - enable only iCloud, Google, or Exchange access to Contacts
On Android devices:
Android Device Settings -> Accounts & Sync -> Connect your Google account that is syncing with FullContact
On OS X:
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.