The red part represents the number of people who play game worlds. The black line represents people who play social worlds. This actually exaggerates the number of people who play social worlds, because the line is a pixel wide; really, it should be so thin that you can't see it.
Now imagine that the black line represents game worlds and the red part represents social worlds. That's close to the way things are when it comes to researching virtual worlds.
That's academia for you.