When in a dispute between two people this is when you go on someone's wall usually with the permission of the other person just so you aren't called a stalker to find something in either a picture or wall post etc. to prove a point.
Can also be used to help a friend find a new profile picture, fan page, appropriate group, etc.
Tali: I am sooooo ugly and fat!!
Aaron: Nooo you aren't you're kinda hot and you're definitely not as fat as other people i know
Tali: prove it!
Aaron: Can i use your fb profile to prove it using facebook evidence?
Tali: Yeah sure
Aaron: Ok heres that picture of you where everyone commented on it saying how gorgeous you looked
Tali: Fine you win