Bits don't have expiration dates. But memories will only live forever if the media and file formats holding them remain intact and coherent. Time can be as deadly to data storage as it is to carbon-based life forms.
There are lots of ways data can die: YouTube can pull a video offline before anybody snags it, your hard drive can crash, taking ultra-rare Grateful Dead bootlegs that you never got a chance to upload to Usenet with it, or maybe you designed a brilliant piece of visual art a decade ago in some kooky file format that simply doesn't exist anymore, and there's no possible way to view the file without traveling to some creepy dude's basement a thousand miles away.
What we're talking about is digital rot—or data rot or bit decay or whatever you'd like to call it—systemic processes which can mean death to data. Kind of a problem when you'd like to keep it around forever. Let's paint this in broad strokes: You can roughly break the major kinds of rot into hardware, software and network. That is, the hardware that breaks down, the formats that go extinct, and the online stuff that vanishes one way or another.
The Hard Life of Hardware
Everything's gotta be stored on something. And guess what? All media age. (Except diamonds—bling bling, biatch.) Brain cells die, film degrades and hard drives break.