I am on an all-girls team and I enjoy it a lot. We do well and we have great dynamics. We are also lucky because we share a lab with an all-boys team so it turns out about equal.
We do a lot of activities with younger girls to try and keep them into science and technology. I (and many of my teammates) were lucky to go to a middle school that stressed science and technology for girls. It even had required CS and robotics courses.
But we find that a lot of girls who go to traditional middle schools stop paying attention to computers and robotics. They are given thousands of messages everyday from their classmates, the media and even their parents about what it means to be a girl, and to be "popular" and "pretty". It sometimes happens in elementary school but it's a lot rarer. We did a fundraiser where we taught 4th and 5th graders about 10 types of engineering. If they had been 6th and 7th graders we wouldn't have had anyone sign up. Instead we had nearly 80.
I think that this is something that we all need to work at. It's definitely improving in FIRST and a lot of girls I know are interested in technology, even if FIRST isn't necessarily their thing.
Also, I heard a rumor that there might be an all-girls regional this year, and I think that might be a good opportunity to see how other teams are doing at getting girls into robotics.