There are relatively few women in computer science and even fewer in some other technical computer areas such as systems administration and in particular communities such as FLOSS. There are numerous hypotheses as to why this is the case, debates as to whether and how this is a problem, and how to change it.
Such a change cannot just happen over night. Even as other barriers to entry are identified and removed, there will be hysteresis in change. This is a general principle which applies to other similar situations of a minority entering a field in which they were once relatively rare, especially one from which they had been systematically excluded. It takes time, perhaps at least a generation, before people fully register that social attitudes have changed and that areas which once seemed exclusive really are attainable to more than a handful of thick-skinned pioneers.
Specifically, as long as gender stereotypes are reinforced and girls are taught (explicitly or implicitly) in childhood and adolescence to shy away from math, engineering, tinkering, and/or geekdom, they will be disinclined to enter these fields. As long as women who are pursuing or already in these fields believe that they are unusual and alone, they will be more likely to abandon these fields for others in which they feel more comfortable.
I think it is crucial that, in parallel with other efforts, girls and women (and parents and allies and really everyone) see that they have role models, mentors, and colleagues. So here is a very imcomplete collection of links as a starting point. It's skewed towards the U.S. and Linux/Open Source because that's what I know best.
(Last modified 2011)