There are at least a few different things to be considered here, and I'm instantly reminded of a big stinker of a post by Mark Pilgrim about proprietary apps and data formats as well as another old article about how stuff that was archived by some government agency was not even readable or accessible later on due to technology changes (it's not one of those referenced in the Wikipedia article for "digital dark age").

Firstly, the medium of storage, the encoding and the interface form one angle to look at. Just like how floppies and CDs are now (almost) obsolete, there would be a future when there won't be any machines that recognize a USB storage device. The same would hold good for other technologies that we have quickly run through, to mention a few - PATA, SATA, PCI, PCI-X, PCIe and so on. Can you read an MFM hard drive today with any computer that you have? With adequate care, data can be moved from one medium to another, like how people learned to move music from tapes to CDs and then to flash drives and hard drives, then to the truly nebulous thing called "cloud", etc.

Next, consider the data formats themselves and the applications that support them. This is where proprietary formats, especially those that are not widely popular, would hurt the users. So if you're using, for example, Apple's document formats on Pages/Numbers/KeyNote, it's likely that those files will soon become obsolete (as they already have, where Apple does not support older formats in the newer iWork). Commonly used and supported formats that don't change rapidly, like JPG and PDF, for example, are safe for a much longer time because there are many applications to process these documents with, both proprietary as well as FOSS. Even the web pages stored by archive.org or any doc or xls files lying around from about 20 years ago - how many more years do you think browsers, word processors and spreadsheet programs will keep bloating up (like they have been so far) just to support older versions of doc, xls, ppt, html, etc.? At some point, the bloat will have to be cut and a decision made that older formats will not be rendered like they used to be. That would leave some clobbered text or gibberish or both showing up for any interested future humans to figure out whether it's worth preserving or not and to convert it to a newer format if they care.

Now, assume the data format is a long surviving one, like say, mp3 or jpg. How do you protect it from bit rot wherever it's stored? Just because you put it on Amazon or iCloud or Dropbox does not mean you can't lose part of the data to corruption of different kinds or lose all of your data due to system failures. Among the people who do regularly backup, perhaps only a fraction of a percentage actually verifies backups (if at all). With consumer level cloud options, there's not a lot of hope for data longevity for non-tech savvy people.

If you're dreaming up a beautiful future in the cloud for all data storage, how can you be sure that your data just doesn't vanish or that it doesn't diminish in quality? People have had that happen to their precious photos by sites that had sneaky terms and conditions about deleting old photos (or if photo prints are not ordered regularly), didn't allow full downloads at any point in time, and sites that went out of business. We can see people treating social networks as reliable cloud storage for their photos as well, disregarding the risks of making such an assumption.

Ignore data integrity and data format issues for a moment, and imagine a distant future where 64K displays are common. All your current and older photos and videos would either look terrible on those or may even be completely indistinguishable. Of what use would these artifacts be then for anyone?

Considering that a lot of data on the cloud is actually insignificant at the level of the human species (like rants and comments on social networks, LOLcats, etc.) and also the huge amount of data being created every second, how would someone in the future even sift through all this? It's somewhat similar to the NSA/GHCQ looking for a needle in a haystack the size of a huge mountain, except that this haystack is to preserve history, culture, etc. The current haystack being built would need a lot of archivists from different backgrounds working continuously to separate the wheat from the chaff and to also look at consolidating (or "packing") the information concisely (like gathering summaries, sentiments, trends, statistics) if we're ever to have an archive that future generations would even want to look at (say centuries ahead in the future). Leaving it to governments alone or corporations alone is not the solution since each would shape the archive in its own image.

This is a very complex topic for most individuals to deal with, and the above points didn't even touch upon cultural and linguistic shifts that happen over time for any data to be usable. I'm sure I've missed many other aspects about prolonging the life of (usable and useful) data.

P.S.: The best everlasting format, as many tech savvy people know, is plain, unencrypted text for textual content (this still assumes that the media can be read, because encoding may play spoilsport).

P.P.S.: All LOLcats may actually be cultural items to preserve for eternity! :P