Social Filtering (Twitter and Facebook)

Do you know who is the guy on the wall in this picture?
He's Hidetoshi Nakata who is very famous person in Japan.
He's known for his notable playing in soccer.
I like his playing style but there were many critics
but the most trustable conclusion is that he didn't get along with
teammates so well....It's just rumor but was whispered into my ear
many time....probably most of time.

This is the real-world example that I would like to talk in this blog....
This is social filtering.....word of mouth...I guess?

I hear this word many times since Twitter came out.
From my perspective, this is "word of mouth done by someone's computer"...by Twitter.

I already have Facebook account and was not willing to use it
but I found that Twitter is very useful.

It is because Facebook and Twitter has different purpose for its nature.

In Twitter, you might have to find someone whose "tweet"(whisper?) has credential with source (URL as usual) if you rely on the info.

Almost all people don't care my "tweet" even there is source.
Some people do because they are my friends.
Many people cares about known famous person's "tweet"
like President Barack Obama even he does not put any credentials on it.

I think that Facebook is for sharing people's profile and meet someone
who you know or may not. It's not made for sharing information there.It is profiling system....you can find anyone's profile to share though.Twitter is for sharing rumors

........but this rumor will be trained many times among people and then it will be truth in a certain time.

I think this training process by people (in a society) defines "Social Filtering"

How do you define the "social filtering"?
Let me know your ideas :)