Yuhoooo....I'm finally home! Yes, I have experienced this problem...over and over.
Here's some "poop"as we call it, and some questions on articles and essay's.
1) Did they "borrow" the article from somewhere online? If so, did the article contain a copyright anywhere on it? If not, the info CAN BE (NOTICE I SAID CAN BE) considered in the PUBLIC DOMAIN. And this makes it usable by others. HOWEVER...if you had a copyright on the site and possibly a statement that said, "These articles are copyrighted and may not be used without written permission ..." bla...bla...bla. THEN...by all means they cannot use the article and are in voilation of the copyright. You can pursue it with legal action if you want to do so.
2. Is this an article that you are compensated for? Are you losing money by them using it? If so, you might want to let them know that by using the article, they're essentially taking bread off your table. Of course you would say it nicely, and always with respect. I will warn you ahead of time that even though you use the best words, and try to be as kind as you can, some people will still be offended that you DARE to ask that they either compensate, give you credit, or remove the article. I've been called everything from a snot to a bad huntin' dog for even asking. Go figure. I finally told one woman that I was coming over to use her car tomorrow. She said, "Huh?" I said, well you don't think anything about using work that brings money into my household, so I'm going to use YOUR car instead of mine, save my gas...and wear and tear. Duh?
3. It might "pay" you (pun intended) to do a simple "google" on your work from time to time. I've found one particular article of mine on 37 different sites. 99% of them were nice and either gave me credit, or removed it promptly. The rest called me names for requesting that they remove it, or give me credit, and promised to send the article out to the world. Sigh...
Hope some or any of this helps. Only you can determine the value of the work and whether or not you feel violated. If the article is serving a purpose and you don't mind it being there, BUT would simply like to be credited for the article, then tell them. I'm betting they will be happy to comply. Maybe. Possibly.
JJ