Only Guys Use The Internet

GIRLS GET OUT

It's a well-known fact that the vast majority of Internet users is male, so that means that you could assume someone who doesn't reveal themselves personally is a guy and be right most of the time. However, women also use the Internet. Hell, one could've edited the very page you're reading right now.

In fiction, this ties in well with Gender Assumption Causes Mass Confusion as finding out someone whose avatar is a guy and speech pattern is very masculine yet turns out to be OF THE LADY KIND could really screw with some people.