i there - yes, Facebook have recently made a change so that all tokens are valid for 60 days. We made the change at our end to display this to the users, and we're also working on a reminder feature to let users know once they approach that date. At that point, you need to login to twitterfeed and go through the "Connect with Facebook" process again in order to provide us with new tokens.
I do realise this can be a pain, but we have no control over this, as it's done by Facebook (and it affects all Facebook apps, as any token provided by Facebook now gets invalidated after 60 days - or sooner, it e.g. you change your password).