I just logged into Facebook on my phone (I’ve only been using it on laptop previously), BUT, it’s not my account.
Now it seems I have full access to someone else’s Facebook account. I’m getting messages from their contacts. 
What can I do about this?? I really need Facebook on my phone now.
Also, do you need to use Facebook to post pictures on Instagram?
Log out and log in.
Should be in top right corner. Then you scroll all the way down and it allows you to log off.
After you log out I would also uninstall and reinstall.
Yikes - Facebook said I logged off. Then I entered the new password Facebook gave me and it took me right back to the same person’s account.
Why reinstall? 
In case the app is storing information.
In that case uninstalling is enough. 
Hahaha clearly I don’t read well.
OK I can get into my own Facebook on my phone now. Thank you for that help! Now I am still in some one else’s Instagram. 
Can anyone tell me how to get out of it??
Logout and relogin…