Many people put a lot of time and effort into getting social media traffic to their websites, only to discover a disappointing fact: that type of traffic doesn't click ads or make purchases! In general, people using social media websites are there to kill time or be entertained; they're not looking to buy.
That's not to say that you can't sell anything using Facebook ads; if your product is expensive enough and your ads are targeted enough, the difference between your cost per click and your profit per item could be enough to make up for the low response rate. However, in this case you're better off buying search engine traffic through Google Adwords, which has a much better conversion rate.
On the other hand, there are some times when you can target so effectively that you easily overcome these disadvantages. Facebook uses the example of a photographer who placed ads targeting engaged women between 24 and 30; by offering a good discount ($500 off) to people who were likely to be specifically looking for that service, they were able to book nearly $40,000 in revenue from a $600 investment in advertising.