Another thing to consider is that in their curriculum, a lot of girls gravitate toward the social studies and English subjects. I was talking to a bunch of my friends the other day, and it seemed that out of our whole group, I was pretty much the only one who favored math and science.
To add, most people think robotics is completely math and science, which it's not. That's why at my school, we've been working to break that stereotype and demonstrate that people who are indeed apt to like the social studies and English more can still get involved in a large capacity.
IA-Howitzer: The intercom thing really works. When 573 first brought girls into the team, that's how they got our interest. Usually if at least one person's interest is sparked, they will talk friends into coming with them (that's usually the case here).