A lot of _comments_ on social networking sites could be replaced with email, however social networking sites have become something of a venue to demonstrate how popular you are - a CV of your social life of sorts. As for private messages on sites like Facebook, I presume that they're so popular because a lot of young people are more in the habit of checking their favourite sites than their emails (I know that I'm more likely to get a response to a message if I send it via bebo or facebook than by email). Maybe it's that peoples' inboxes contain mostly confirmations and other detritus that's driving people to places where they're only going to receive correspondence from real people. Or maybe it's the "OMG Ponies!!!" effect of being able to customise something to reflect the façade you'd like to exhibit.