If you think someone other than yourself has been signing into your Facebook account, you can find out by logging into your Facebook account from your iPhone and then tapping on More in the bottom navigation. From that point, scroll all the way to the bottom of the screen and choose Account Settings.

Once you are in Account Settings in Facebook on your iPhone, tap on Security and scroll down to Active Sessions. This is where you can see and end all the sessions that are active for your Facebook account. If you notice that there are sessions logged in from different locations or devices, make sure to change your password as well to prevent unauthorized people from logging into your account.