What are the advantages and disadvantages of social websites like FaceBook, Fotolog, Messenger, etc?

Nowadays, there are a lot of social websites in which if you don't registrate you don't have a social life. But I think that it is not necessary for young people.
It is clear that using the social websites has a lot of positive points. You can meet new people from different countries. You can talk to your friends, see and comment the photographys of your friends.
But things have always a negative part. Did you know that everybody can see your name, adress, telephone numbers, download your photos, know who your friends are, etc. The social websites normally have a low nivel of privacity. Another problem is that many people become addict to the internet and they can't sleep, eat because of the websites. And at the social website you never know who is on the other side, with who you are chatting. For exemple, one day on a page like FaceBook one man wrote to me explaining that he enjoyed having sexual relationships with the children describing what he was did with them and asking if I'd wanted to share it with him. It was horrible, but it is a normal thing at social websites.
In my opinion the people had better to have a social life but in the real life and not at internet.