I really think Fincher has had like a three act performance. Seven, The Game, and Fight Club were Act One. Panic Room and Zodiac were Act Two. The Social Network was Act Three. Really three distinct "eras" if you will.
The first three were just really really new and unique (though I felt Seven was quite overrated. Brad Pitt was quite petulant and, if not for Kevin Spacey, that movie probably falls apart). Fight Club just really was like a two hour gut punch (no pun intended). Narration in a movie is hard and god damn if Norton and Fincher didn't just nail it. The Game just was a great time for me. I saw it a few times in the theatre and just had a blast with it.
The next two felt like Fincher cashing in. I enjoyed both movies (Zodiac especially because I had read the book like a hundred years ago) but they were jam packed with flaws and what felt like settling. Panic Room had bad villains and a protagonist that you hardly cared about. Cool concept though.
Man though...The Social Network really knocked it out of the park for me. I remember seeing the previews and thinking "How the hell are you going to make an interesting movie about Facebook?" I honestly can't think of a single thing wrong with that movie though. The pacing is excellent. The music works perfectly. For a movie about computers and coding...getting Trent Reznor and Atticus Finch in there seems like a perfect union.
I'm a huge huge HUGE fan of David Fincher so I'm not exactly without a bias here, but I'd say The Social Network is his best movie and it ain't even close.