Is it still possible to track a stolen iPhone 6s Plus eventhough it has already been restored to its factory settings?
Find my iPhone is a feature that cannot be disabled unless a user has the password to the iCloud account, and when the phone is in lost mode Activation Lock would kick in should someone restore the software on the device (rendering it unusable). The application being on the phone, however, does not necessarily mean that it was enabled for use. Consider the following scenarios for your phone:


The phone had Find my iPhone enabled and is powered off/no longer working
Find my iPhone was not enabled on the phone, and the thief was able to restore/wipe the device
Find my iPhone is enabled, but carrier service has already been suspended to the device, meaning it cannot get online to post its status and to check the lock/lost status


In any case, it is impossible for us to determine what chance you will have of retrieving your device. It is always recommended that you submit a police report with a high level of detail, including the phone serial number, so that it may be traced back to you should it be found. Also, should a user try to sell it to someone else or take it to a carrier for service, it's possible that they might see it is in Activation Lock and let Apple know about the device. Beyond that, it is generally considered a lost cause.