You have to trust that these companies will delete your data when you ask them to, but they're generally not interested in doing so. Sites like these are more likely to make your data inaccessible than they are to physically delete it. Facebook is a known culprit: actually deleting your data from its servers requires a complicated procedure that may or may not work. And even if you do manage to delete your data, copies are certain to remain in the companies' backup systems. Gmail explicitly says this in its privacy notice.

This reminds me of Jonah Lehrer's description of our brain's method for deleting. He says that we don't actually forget things. Every single memory remains chemically encoded in our brains, which are virtually limitless hard disks. What happens is that we forget how to find our forgotten memories.  (It's kind of like me with my keys or the ear-phones to my iPod. I don't actually lose them. I just don't know where the hell they are...)