If you work as a photographer long enough, your pictures are going to be used without your permission somewhere. Tracking down where that's happened can be tough, but this great video will help you in that endeavor.
I'm personally not excessively stringent in pursuing every instance I find of my images being used without my permission online. It would be far too time-consuming to do so, so I generally just roll my eyes and move on. Where I do draw the line, however, is when that usage is in a manner that should either be generating money for me or is misrepresenting someone else to generate money for them. However, even just tracking down commercial misuses can take a good chunk of time, so it helps to automate the process a bit, as Anthony Morganti demonstrates in this video. One great trick is setting up Google alerts for your images or even using a paid service that will allow you to apply imperceptible watermarks to your photos and then crawl the web for them. In the end, it comes down to balancing how big a problem it is for you vs. the amount of effort you want to put in to rectify it.