So after some encouragement and looking at the plagiarizing shitstorm that is Wattpad, I decided to dig a little deeper and turn up a list of candidates where you will likely find your story being posted without citation or plagiarized. I have also created this blog as a means for drawing attention to a petition made to prevent this from happening again. Please feel free to sign it and get administrators to realize this is a bigger issue than they think. The more signatures the better.
Let's get the obvious out of the way first, Wattpad seems to have a pretty large issue with not patrolling its sources. I had to request two authors to take my stories down after they refused to cite my work/tried to remove my comments asking for them to cite me as the original author or straight-up changed story information to avoid detection (which comes off as odd as the others cited me without issue and I didn't have to bother staff to get them to intervene). One author even claimed to have written an incomplete version of Your Secret Admirer and The Call of Cthulhu which seems to highlight the lack of patrolling going on there if H.P. Lovecraft's story can be posted without detection for months at a time until a whiny user points it out.
Quotev seems to be another likely source for turning up copies of your story without citing the proper author. I would strongly advise authors to skim the site and try a few google searches to make sure they are being properly credited.
DeviantArt also has a tendency to post content that may not be properly cited. While I haven't turned up much in my searches here, I have noticed a tendency for popular stories to be re-posted without providing any links/credit to the original author so it's worth a look at least.
So there you have it. Please note that these aren't the only places you'll uncover plagiarism or authors not crediting, these are just the most likely candidates. The more diligent you are in your search, the more likely you'll turn up instances of people copy/pasting your work without citation. Please feel free to use this as a means for searching for instances of plagiarism and voting on a new petition to try and enforce more diligence from the administrators of other sites. As a final note, let's approach this as a community. If you come across someone ripping someone else's content without crediting them, let them know. As a community we can really curb these issues.