For the Facebook app I have turned off background app refresh for it and allow it to only use location services when in the app (if its on always it is ALWAYS updating your location to it). This helped heavily reduce the battery usage while still being able to use the app vs. the mobile website version of it.
This worked for me, too.
I'm not sure that 'only use location services when in the app' is fully functional, though. This morning, after leaving the app, I noticed the location services icon was still it up. I used the app-switcher to kill Facebook and it went away.
Give it some time, your new phone is likely indexing, downloading backups, getting everything setup. The battery also will take a few cycles to be optimized. And it's a new toy, you probably use it more.
6s+ has a smaller physical battery capacity than 6+, so doing things with screen on all the time will result in less battery time.