How can you eb certain?

I just watched some of You've Got Mail, and it go tem thinking. Is it really possible to love someone that you've never met face to face? Can you maintain even an online friendship with said person. Because, for all you know, said person could be an axe murderer. Only you never would have known that because you've only been talking to him online. How can you rally just trust someone so much that you don't even doubt, even for a second, that there's more to this person than meets the eye? How can you really EVER know someone?