I'd like to point out that I believe what you're saying is technically incorrect - at least as it applies to Facebook. It is my understanding that when you ask Facebook to "delete" your content, that is (or at least should be) exactly what happens. From Facebook's "Statement of Rights and Responsibilities"[1]:

1. For content that is covered by intellectual property rights, like photos and videos (IP content), you specifically give us the following permission, subject to your privacy and application settings: you grant us a non-exclusive, transferable, sub-licensable, royalty-free, worldwide license to use any IP content that you post on or in connection with Facebook (IP License). This IP License ends when you delete your IP content or your account unless your content has been shared with others, and they have not deleted it.

2. When you delete IP content, it is deleted in a manner similar to emptying the recycle bin on a computer. However, you understand that removed content may persist in backup copies for a reasonable period of time (but will not be available to others).

From their Data Use Policy[2]:

When you delete an account, it is permanently deleted from Facebook. It typically takes about one month to delete an account, but some information may remain in backup copies and logs for up to 90 days. You should only delete your account if you are sure you never want to reactivate it.

And, finally, from a Facebook employee here on HN[3]:

Maybe that was true in the past, but today when you delete your data it is gone. Trust me, I wrote it myself. The law enforcement guidelines that have been circulating recently corroborate this.

I believe the above quotes demonstrate that Facebook, at least, do have a legal reason to delete their copy of your media when you no longer want it hosted there -- and that they actually do so. Maybe that will change in the future, but I hope not.

Personally, I think it is dishonest to use the term "delete" when you simply mean "remove this reference to", and I appreciate Facebook distinguishing between account deletion and deactivation. To most computer users, delete implies that the thing is gone and can't come back. Using that term to mean something else is deceitful. As such, I believe sites that allow users to "delete" their content do have a moral obligation to actually go through with that and delete it entirely, not just make it invisible to the user.

It is my view that once a service becomes culturally entrenched, it becomes accountable/responsible in many ways to its users. This attitude of "well, if you don't like it don't use it" is an interesting one (and it's something I adhere to for the time being), but I wonder how long it can last. Recent SOPA outrage demonstrates that people generally don't have this sort of attitude towards governments. The differences between the two are clear, of course, but there are similarities too. I find it interesting that people[4] will condemn, for instance, full-body scans at an airport as being an invasion of privacy but then have this cavalier attitude towards corporations like Facebook - hey, you agreed to the terms and conditions! At what stage does that agreement become entirely meaningless?

[1] http://www.facebook.com/legal/terms

[2] http://www.facebook.com/about/privacy/your-info#deleting

[3] http://news.ycombinator.com/item?id=3320240

[4] Not you, dotBen, but people.

EDIT: I'm sorry to see you've been downvoted. I hope people aren't doing so just because they disagree with you.