All but two of the sites I'll be discussing are imageboards. Imageboards are near-live conversations between many users, some providing content and others commentary. Many ancillary websites, such as rapidshare and megaupload, provide hosting for larger collections of content, which are linked from the imageboards.
The mother of all imageboards, 4chan's "Sexy Beautiful Women" board is a rapidly-changing mass of pornographic media. Though only five pages deep, the number of images and other media linked at any given time is impressive. If you see something you like, grab it quickly. Odds are that thread will be gone within 10 minutes.
Like all imageboard sites, the content comes from the users. That's not to say they're creating the pornography themselves, merely sharing pornography they've acquired from other sources. The content persists only so long as new content does not push it off the board. This process democratizes the popularity of pornography, similar to how digg users promote articles.
7chan hosts a variety of pornographic imageboards and has recently taken over for the inadequately hosted fapchan. While functionally very similar to 4chan, 7chan allows users to find and share pornography more specific to their tastes by providing fifteen individual boards.
While 7chan also sports a "Sexy Beautiful Women" board, they also provide a companion board for "Sexy Beautiful Men", as well as a variety of fetishes, comic styles, and even an erotic literature board.
What's the password? Hell if I know!
While 7chan provides a better categorization mechanism for pornography, anonib takes this one step further. The boards making up the site are also user-created and administered, thus fully democratizing the entire experience.
The pornographic boards are one NSFW click away from the main page. They're ordered with the most recently updated boards listed near the top. Even the boards themselves have sub-boards, which don't get as much traffic as the main boards. The variety of content is enormous. Rule 34 is alive and well over here.
This is the adult-themed answer to YouTube. Users upload videos, which other users watch, rate, and discuss. While most of the content doesn't seem to be user-generated, it's still a step towards bringing makers and consumers of porn closer together in a larger community of porn-interested people.
While not explicitly a porn site, Stickam is a great place to see those "real live girls" you see advertised in so many banner ads but never manage to find even after clicking. It's a video-chatting site for anyone with a webcam. Find a girl with low self-confidence or something to prove and you'll get all manner of naked in no time flat. It's not uncommon for users of the aforementioned imageboards to raid stickam and post the results of their exploits for all to see.
I hope you found this article informative. If there are any other social pornography sites you feel should be included, post them in the comments and I'll add them after I've done some more research.