All of his films do deal with isolation, which I'm surprised no one has mentioned in favor of this idea that he's an auteur. Either way, along with his bleak outlook on society, focusing on subcultures and outcasts - that's all inherent to the scripts. Those are things that are central to the plot and that the writer often times makes very clear in establishing the tone of their stories. To approach that as the director as an auteur is silly when comparing it to something like Kubrick throwing sexual metaphors into his movies when his source material had none what-so-ever.
The darkness described in The Social Network, it's attributed to Reznor justly. I said when bringing this all up initially that Fincher has good taste in music, but that doesn't mean he's an auteur. I could reedit The Social Network with the score to Kramer vs. Kramer and it would change the tone, and in general, I doubt most of the movies fans would become nonfans. Scorsese uses great music too, but that's not what makes him an auteur; that's a director doing their job.
Picking good subject matter, having a strong visual style and making images mesh with music are pretty much all the strong traits of a music video director. Fincher isn't making music videos, he's directing feature films. He does a great job, but he's not making films, he's directing them.