Facebook app nagging about syncing. Can it be disabled?
By request from 'Dan Hulme' who replied to my question, I got the message to pop up again and managed to get the app-info. It turns out to be an app connected to the Facebook app called 'SNS'. I simply removed the tick from the "Show notifications" option. The message no longer pops up.

It can otherwise be found on "Settings -> More -> Application manager" Shuffle so you see the "All" list, then scroll down til you find the SNS application.