It still doesn't mean we can't just "delete" it. It only means it may be difficult to completely eradicate what you wrote at 3 a.m. in the morning, drunk, bitching about your girlfriend who just dumped you.
That's the darnest thing about the Internet or social networks, especially places like Twitter or Facebook: you just don't know who is reading. Sometimes you don't really think, and you don't remember that your tweets are archived and broadcast to everyone (not to mention "retweeted"), so you'd better be careful when you talk trash about somebody or some group of people. Words (or in this case, texts) travel really fast on the Internet. And friends of friends of friends could see what you posted on your Facebook status updates, and they may turn out to be in the group of people you just badmouthed. Now, granted, you may not want to be their friend anyway, but still, you have no idea who they will talk to, or who they know. Perhaps they are your boss's best friend?
The idea is, if you don't have something nice to say, don't say it, especially on the Internet. And that goes with doing it anonymously -- so many people nowadays hide behind a fake screen name and they spew whatever garbage they can think of. First amendment right without the responsibility, that's what the Internet has become. And if you're silly enough to post something derogatory under your own name, well, then don't be surprised if it comes back to bite you in the ass. People have been known to get fired over what they posted online.
To me, that seems harsh. But the line between what is private/personal and what is public is really blurry, and as far as the Internet is concerned, whatever is Googleable is considered public anyway, so we do need to be careful of what we say online. Bitching about your boss on your blog may not be a good idea after all.
Then again, there's that "delete" button.