Where did all these photos and videos come from? My phone's memory is so full that it won't let me take or download pictures anymore. Also cannot download almost any app due to low memory. When I look at memory usage, most of it is used for photos and videos. I plugged my phone into my computer and imported all photos and videos. I then would delete a bunch of them off my phone because they were now saved somewhere else. When I looked at what had been imported to my computer I saw hundreds of photos I have never taken or viewed before. Most of them were of people I know but have never taken pictures of. Also there were several video advertisements for brands like Colgate, McDonalds, and Gucci. I cannot find these photos or videos on my phone, or else I would delete them, they are almost certainly the reason for my phones low memory. All I can figure is that they must have come from Facebook. How do I find and delete these photos/videos, and also how do I prevent them from being saved to my phone?

Any help with this is much appreciated!
It sounds like your facebook app is keeping a cache somewhere on your phone. Using an app like SD Maid, or any other phone cleaning application that searches for and deletes cache files such as this should solve this.