Someone else's Facebook messages are being received on my iPhone...

Hello,

I bought an iPhone 5c and I believe it was once in the possession of a Lithuanian as his iCloud was once connected to the phone. Now since he removed it, I have added my own iCloud account etc but have found that (it seems) that I still receive incoming Facebook messages in Lithuanian, which I must presume are due to him being signed in on Facebook on that phone at some point. The phone didn't have the FB app when I got it, so I downloaded it myself and have signed into the app, as well as in the Settings area so I can't work out why it is popping up with notifications of his incoming messages? Can anyone offer direction? Many thanks!