Howdy ho! If anyone else has a post like this, please let me know, and feel free to share as there's really no way to compile a list of all possible writing sites' report pages, so I'm going to hit the big ones that I know of where plagiarism is a thing and can happen.

So you've found a story that's been plagiarized: yours, a friend's, a well-known author's, etc. But it's not on AFF, where it's easy to report content by way of a button on posts. It's on wattpad, or tumblr, or livejournal, or somewhere. And it's your story here on AFF, or somewhere else. How do you contact the website administrators and get things sorted out? Well, here are a couple of helpful links (for me to use as well lol) in the event something awful like that happens. I can tack sites on as they come to mind, or as you guys tell me where you've seen stuff reposted a lot.

REPORT CONTENT ON:
Wattpad
Tumblr
AO3
Livejournal

Blogger/Blogspot: done through Google Legal Help (also applies to YouTube)

deviantART requires you to have an account (which is free of charge) in order to file a DMCA complaint with the site using their form; otherwise you can mail the complaint as a PDF with the subject [Plagiarism] to [email protected].

fanfiction.net: send a email with the subject [copyright violation] to [email protected]; the process of filing a report can be found here.

Please note that these sites all take plagiarism and copyright violation accusations very, very seriously; a plagiarism accusation proven true can have an account removed. I cannot stress enough how important evidence is. Unless you have sufficient evidence that what you are reporting is indeed yours, do not abuse their report links. AFF works differently than these sites. We all take the subject seriously, but these are much bigger sites with bigger staffs having to look over a ton of reports each day, and they will not waste time chasing a "maybe" or "possibly." Screenshots, links, physical proof that your copyright was violated...provide them, guys! The more evidence you can provide, the more solid your case becomes, and the more likely they will take down the content.

On a sidenote: do not put people on blast, not here on AFF at least. It could be seen as hatemongering and can turn into something very messy, and the moderators can step in at that point and deal with any violations or rude users. The minute you put someone's username or link to their profile, anyone who reads your posts (friends, readers, whoever) can now harass them, possibly even bully them, when you have good intentions in mind and only want your story taken down. I know how disappointing and discouraging seeing something of yours copied is; it's happened to me a handful of times, and you do get so very angry and want something done then and there, but don't let things get out of control. It's hard to backpedal at that point, and you're somewhat accountable if you can't get a hold on stuff. Use the report content button in that case; believe me, they all get seen and handled.

Do the right thing, report copyright violations, but don't be a jerkwad about it all. No one likes a jerkwad.