Nowadays companies have a lot of information about their customers. If you belong to a grocery store chain's loyalty program, the store chain probably knows your demographic information, and also what you bought, how much you bought and when and where you made the purchase. All this information can be used in market segmentation.
The most common market segmentation method that I see applied every day is browsing-based ad retargeting. Web media has been doing this also for quite a while.