I really don't think the grandparent meant that Facebook is a subculture. I think he meant that startups and founders are the subculture and that Facebook is a result of that culture.
At it's core the plot is about a pretty specific experience (the challenges of founding a tech startup as a young, inexperienced hacker), but the emotions and the drama are universal, which is why I think the movie will enjoy widespread success.
At the end of the day, though, it's about founders and hackers and startups. And that's awesome.
I was actually wondering how well the movie will do because, while it's about a service used by 500 million people, it's not about the service itself but about the creation and early days of the company. So a facebook user might not necessarily be interested.