Deleted facebook messages comes back ???

[tooms]

Hi I have an Iphone 5s running newest IOS7. I have the newest Facebook app installed. My problem is; I delete facebook messages on my phone. But when I then use the new "close running apps" function on my phone "doubleklick home button and swipe up". When I then restart the Facebook app the deleted messages are back ??? They do not apear, when I log on my iMac only on the phone ?? Whats happening

Best regards Tooms