It's a textbook example of Prisoner's Dilemma - we all realize that someone needs to get on the empty servers, but no one wants to be the one to do so.
Let's illustrate - let's say that in a timespan of 10 minutes, 30 people hop on game-monitor to check the FS servers. They find empty servers, and only three of them hop on. The rest of them see 3 people on, causing some of them to join - but the server still only gets 5-10 people or so.
By contrast, if all those 30 people hopped on, we'd have a great game going. Instead, we have a mod that doesn't get played as much as it deserves for no better reason than that it it doesn't get played as much as it deserves. An evil circle in my eyes.
If everyone hopped on servers even when they were empty, we'd have loads playing. But people don't want to do it, creating empty servers.
"One of the bitches actually gave birth while she was attacking, and her puppies joined in on the carnage."
--the awesomeness that is Boatmurdered.