...I would hazard a guess of 25%, but I really can't say. I tend to mentally filter Fangirl when I see it on my flist, which is to say, I'm half blind to it. Also, some of them don't type the way they talk, so I can't really confirm that. I have met people at cons (rather than meeting them online for the first time), and lots and LOTS of girls seemed to be talking Fangirl there. But cons are unusual circumstances---they might have been doing it to show off for a special occasion, or they might have just been affected by the weekend-long marathon of Japanese stuff coming their way. And then there are the degrees: do they just let a "wai" slip once in a while, or is it "-chan" and "ne" and cat references in every sentence? Something in between?

I think the proportion isn't really something that spreads across everybody who's into anime. It varies from group to group, fandom to fandom. Everybody seems to agree that fangirls are social, so I think they take their cues from others and act as a group. If they're in a community or group of people that doesn't mind Fangirl (or tends to speak it already) then a LOT of them will speak Fangirl. So if you're with certain people, it will be omnipresent.