Why is my Facebook account logged onto multiple devices?

I was just randomly checking my account settings and was looking at devices that are logged onto my Facebook and it shows multiple devices, all from iphone 7plus which is what I have. This might be a stupid question, but wouldn't it just say iphone 7plus once if my phone is the only one logged onto Facebook? On my account settings, it shows 5 other iPhone 7 plus. Does this mean someone else is using my Facebook? I'm just really confused.

IPhone 5 or iPhone 5c?

Which one should I get? Which one do you think has a better battery life because that's a major problem I'm having with my really old iPhone 4s.