Friends are the players you’ve added via Game Center or Google Play as well as friends on Facebook. There’s no limit to the number of friends you can add through these services.
Friended Trademasters are players you've added by visiting their islands via the World Market (as of Update 1.3). You can add up to 8 Trademasters using this system.
The above is Quoted in part or full directly from the Official Paradise Bay FAQ