If you’ve posted online and it’s truly yours, then it’s officially yours. All genuine content published online is immediately protected under copyright
What it means is that anyone who takes your content and pastes it on their own site without permission violates copyright law. If someone takes an image from your blog and posts it on their own blog, that’s copyright violation
fair use clause states that people have limited rights to use your content as long as the use of your content is deemed ‘fair’
Unfortunately, there’s no clear cut definition of what exactly fair use is.
Anyone in the world can copy your online content and insert it onto their website. After making some changes, one can claim your content as his/her own. The web is built on work of people who spent their time and energy to create original content. Global epidemic of content theft violates people's rights and discourages the creation of new web content.
One thing that is not protected under fair use is a mirror copy of large portions of your content used for commercial purpose. If you find full-text copy of your content published on other web sources without your permission, you have a good basis under copyright law to contact and claim your rights of exclusive use.
Whatever worth it may be, you may initite legal action by first serving them a legal notice on this and demand the relief that you expect after which you contemplate proper legal action as per provisions of law.