Mike on Ads has a cool script to infer whether a user is male or female, based on browser history. If you have it analyze your history, it will give you a list of the sites you visited and the corresponding male:female ratio. He got the sites to poll from quantcast, but I can't tell if the demographics came from there as well. The numbers seem to be different when I plug them in, so I'm guessing he either used more/older data than what's currently up, or got it elsewhere.
Here are the ratios for various social media I plugged in, in order from "most male" to "most female":
Site M:F ratio
Twitter and Wikipedia didn't seem to be in the feature set. However, straight form quantcast it seems Twitter's ratio is 0.97 and Wikipedia's is 1.07. Quantcast also lists a ton of other demographic info, which is interesting to look at.