If you sell any sort of product or service online, you should be using Facebook retargeting to bring back customers from Facebook and web retargeting to bring back your lost customers across the web. No matter how great or well-optimized your site is, more than 90% of the people who visit will leave without signing up. With a Facebook or web retargeting campaign, you can keep your products and services in front of those people after they leave, and eventually bring them back to convert.
The coolest thing about both forms of retargeting is that they work! Unlike every other form of online advertising, where you spend $X and see some fraction of that come back as sales, with Facebook and web retargeting, you can advertise profitably. On average, our clients get $8-10 in sales for every $1 they spend with us. It works so well because we're advertising to people who already showed an interest in your product and just need a reminder to come back. It's a brutally effective form of marketing.