It is commonly said that the human body replaces its cells every 7 years. It is less commonly known that 98% of the atoms of your body are replaced every year. The actual absolute truth of either notion is probably disputable, but they are close enough to serve my point. After a few years, virtually everything that made you you is gone. This leaves an intriguing question: what exactly are you? While this is also an interesting philosophical question, I'm really working on the more practical angle. Here's basically the answer: you are a low entropy depression in the universe. You suck good quality energy from your surroundings and spit it back out as heat and waste all in an effort to... exist. Thinking like a universe, you are essentially a weather phenomenon, a low-pressure front.
I consider this interesting because you aren't really this well-defined thing that exists from birth until death. You aren't so much a solid object but a place in the universe where things are flowing in and out in some orderly way. The primary difference between you and the weather, then, is if you are fit, in the natural sense, you might be lucky enough to spin off a few thunderstorm copies of yourself. Through this incredibly ephemeral existence your genes are passed down. So even though the atoms are replaced continuously, the pattern in your genetic code lives on. Your genes are potentially immortal.
Immortality may be a strong word but if there is anything of this Earth that deserves the title, it would be our genes. The most stable (and important) bits of genetic code can have their lifespans measured in the hundreds of millions of years, possibly billions. To put this into perspective, our universe is about 15 billion years old. In your DNA is a self-replicating pattern that has existed for a non-trivial portion of the history of the universe (where 150 million years represents about 1% of the age of the universe). That's pretty impressive for a bunch of carbon chains.
With all of these physical pieces of the internet being constantly replaced, it still comes as no surprise that the information is preserved. Sites like Google's cache and the Internet Archive have saved large sections of the internet. The biggest difference in our little analogy is that the Internet and its information, unlike us, is not self-aware and probably won't be self-replicating any time soon. As long as we keep those hard drives from failing, though, the information contained therein is going to live on.
I saw one of my recent posts in Google cache. This got me thinking. Like the billion-year-old bits of genetic code in my DNA, how long could I reasonably expect my post to live? What about something that's been saved in the Internet Archive? Do you think its a stretch to say it might live as long as the human race? I don't think it's a stretch at all. The Internet Archive project might run out of money or suffer some catastrophic data loss, but chances are good that someone will step up to preserve the archive and/or a backup exists somewhere to replace the lost data. As time goes on, it will become increasingly more important to save this for historic purposes. When does it end? Maybe never.
If you don't already know the term, now is a good time to refer you to Wikipedia's entry on memes. Memes are units of cultural information that are passed around our collective consciousness. Memes in the cultural world are equivalent to genes in the biologic world. They undergo natural selection, evolution, and all the other tenants of Darwinism. The fittest survive and are replicated. The bad ones go away. The internet has given memes the type of immortality previously only afforded to genes.
Every time you and I post to the internet, someone somewhere might archive it. And those archives will keep getting saved. Saved and copied and saved again. These archives may seem, now, to be only a novelty, but their importance as a historical tool will only grow as time moves forward. They will become increasingly more sacred. It's kind of funny to think about historians digging through the early Internet Archives and writing a paper on the Rick Roll meme. What's interesting, though, is that I'm pretty certain almost every participant in these "early" internet times (ie, us) is going to leave some mark in that archive. That means every time you write something, it might get saved for the rest of humanity to read, ever after. I don't think ever before has every person had this much access to this type of longevity. If the Internet is immortal, then we in it shall be remembered.