Sometimes the answer to a number of web design questions is "well what is company X doing?" -- the assumption being that these are smart companies who might even be doing A/B testing with a lot of thought and metrics. So following FB's lead, it's a decent practice to request a web browser at least 1400 pixels across to show everything? To me that implies they think people tend to run FB maximized, or at about that width - relatively few screens are near double that.
Of course, there was a quiet revolution in the 2000s that tended to abstract literal pixel resolution from what every program feels it's working with. Compared to some early attempts in the first half of that decade, the results are astonishingly transparent to users and developers alike.