Rather
rare, similar to
girl geeks, and there is
likely a lot of
overlap between the two.
Even more rare are hard-core girl gamers, the ones who really like a large variety of games, and aren't afraid to admit it. They're the ones you see kicking some behind on Quake, or otherwise getting that kind of attention.
Many girls who play online don't like making their gender known because of the hordes of adolescents who somehow think they have to treat them differently, or that they're somehow special.
I'm one of that group. I suspect it had something to do with both of my parents being into games - I spent plenty of days with the old TI-99/4a computer working on the Scott Adams Adventure games, or the Infocom classics, and later on we fought a little (though in a friendly way) over who got to play the latest RPG on the Super Nintendo.
I also seem to notice there are very few girls who play in Nomics, whether here, or anywhere else on the internet.
One thing I've noticed about us when playing net games like Diablo II is that we seem to do better than the guys at that whole cooperation thing; we "play well with others". Every single time I've played in a party instead of by myself, and found it worth my time and effort, has been with other girls. Maybe it's just due to the abnormally high number of teenage morons doing all that stupid stuff that's got me thinking this way, but the only players I've enjoyed working enough to write their account/character name down for playing with later happened to be estrogen-based. Maybe it's the tendency to share fairly, not be a hero and run ahead and do it all, and the fun little chatter during playing that does it...