So how do you use this face: >_>?

Urban Dictionary tells me it's a glare, or an angry face.

I would use it as more of a nervous face, or "shifty eyes".

I can be used as either. Usually "><" is more of an "OSHT" or "DAMNIT" face, while "<<" or ">_>" is either shift-eyes or, like urban dictionary says, glaring. There's no set meaning for it, so it can really be used in any way you choose.

I use it as a way to glare at people over the internet.

I use it to easily tell which posters are feeble-minded idiots.