I think that one can find the social impact of Del Toro's work in the message it sends out to the audience.  Consider Del Toro's understanding of the fantasy or fairy tale genre, in general, as part of this impact:

Much like fairy tales, there are two facets of horror. One is pro-institution, which is the most reprehensible type of fairy tale: Don't wander into the woods, and always obey your parents. The other type of fairy tale is completely anarchic and antiestablishment.

Given the critique of the Franco Regime in Spain and its authoritarian and brutal means of control, as best embodied by Vidal in the film, the social impact of the film is one in which individuals must question their authority structures, especially when power is being abused in the name of "authority."   This is seen in the spirit of freedom and courage intrinsic to Ofelia's characterization.  Ofelia's sense of self- sacrifice and commitment to what should be is in stark contrast to Vidal and what is.  Through this, there is a strong statement on elements should comprise human action in society, representing a social impact delivered through the film.  The film did not spark revolutions nor overthrown dictators.  Yet, through its release and its worldwide embrace, it is evident that its message resonated and had a significant impact on how people viewed the role of film and society.