Good "word of mouth" equals staying power.
In a month, it is possible we will hear Feige's view on the FF's performance and its imagined effect on Marvel films going forward (although one can guess Marvel Studios is hoping beyond hope that RDJ will be the spearhead of a course correction for the studio).
Regarding word of mouth and Superman, some have posted the hope it will reach 670 million (a reference to Man of Steel's $670,145,518 worldwide earning), but the film currently sits at $583,184,195, meaning it would need to earn another $86,961,323 just to match Man of Steel's earning and after so many weeks in release, that seems a highly unlikely outcome.