The film is nominally about Facebook. I say it's about Facebook because it is - it's based on a nonfiction book, and seems to cover the history very well so far as I remember. And I say "nominally" because, well, it could be about any number of people that started small, achieved success, and got catapulted into situations where they could fuck over people who they'd been calling best friends the week before. Heck, that's pretty much a trope - it's almost the plot of Boogie Nights, to pluck an example out of the air.
What it does best is illuminate characters without ever explaining them. Nobody here really seems to understand themselves, or those around them - the things it feels closest to, in fact, is Mad Men. And in line with that we never really get to see inside the character's heads. The closest we ever get to explanations of motivation are the two speeches, both given by women, that bookend the movie. These feel like the voice of the writer, fed up with the unpleasant atmosphere of misanthropy, sexism, and sociopathic capitalism that run through the rest of the movie, pointing out that the world that the characters inhabit is, in its own way, a niche. I don't have to live in the same world as these characters, even if I do get to use the software they write.
For that I should be grateful.
(Thanks to hfnuala for pointing out that the movie was bookended by decent female characters, unlike the body of the movie, where the characters do their very best to surround themselves with pretty things that they don't have to think about, much less engage with.)