Hi,
I've not noticed any unusual warming up of my iPhone 6 plus whilst using Twitter / Facebook or pretty much anything else.
Can you clarify "Warm"
The iPhone will warm up as it's working and from the heat from your hand.
Try resetting it by On the iPhone hold down the Power & Home button together until Logo shows then release both & the iPhone will reset
If your concerned then book an appointment at your most local Genius Bar.