There are relatively few women software engineers 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 minorities entering fields in which they were once relatively rare, especially (but not only) ones 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 and appropriate for more than a handful of unusual 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 these fields are overwhelmingly male, they are likely to evolve cultures which are comfortable for those men already in the fields, but not necessarily welcoming to those who outside those fields. As long as women who are pursuing or already in these fields believe that they are unusual and don't fit in, 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 incomplete (and possibly dated) 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 2013)