There is a popular saying, "actions speak louder than words".  It means that what people do (behavior, actions) tells you more about a person than what the person says about himself.  If this is true, and I believe it is, then knowing someone online only by what he says, tells you very little about the real person.  To see this person in action, among friends, with family, in society, can tell you if what he says to you about himself is really true.  It is very easy to say things, some people can lie very comfortably.  I am a very honest person, but it does not mean that everyone else is honest when they talk to each other. 

Knowing someone face to face and building bonds is very important.  It is not to say you cannot make really good friends with someone over time through letters or online.  I have done just that with a few people.  It just takes a long time and finding someone really honest who is willing to open up to you. 

To Erica:Thank you for your answer.I agree with youin most situations.:-D~

I do think real friendship can stem from social network websites and online interactions. However, it's important to be wary of catfishers. I agree with what Stephanie said above, and personally believe that real friendships can exist through online interactions on the basis of trust and open honesty.

To stephanie,thanks for your answer,quite a logical and reasonable demonstration！I agree with you that someone may fake their imformation and we really know little about them and have no way to verify.But as you said,there are always some kind and honest guys just like some foreign friends I recently made on italki.There is a piece of poem I want to share with you which was written by a famous poet called Bo Wang in Tang dynasty of ancient China."海内存知己，天涯若比邻".Its means though some bosom friends of yours live far away from you,you are just like neighbours because you are close psychologically.I think it is pretty appropriate in imformation times now.O(∩_∩)O~