This is not discussing the entire internet, but trends among specific services designed make it far easier to add your personal data than remove it.
And arguably, your comment is not objectively true, but only tends to be true. There's no technical reason why data (even personal data) can't disappear from the internet - one only need look at the recent worries about the state of repos on Google Code to see that, until there's some overriding interest in backing something up, it doesn't necessarily get backed up.
