Today I came to know that typing "Facebook" in Google home page giving suggestion as hyper link. (i-e) Clicking on this link will take you directly into the facebook.com website.
For twitter, reddit,w3schools, stumbleupon and cnn also it behaves similarly.
I have tried it by typing yahoo, Microsoft, bbc and myspace. But it didn't work for them.
If anybody knows more details about this feature you can share it here.
It seems this feature is available from May 2009 itself.