You may have heard about retargetting or remarketing. This is the simple process of getting a user to click on your ad, tagging them using a facebook pixel or google adwords, then continuing to advertise directly to those people.
A few days ago I made a post about the differences between Google AdWords and Facebook ads, and how they might work differently for different niches. But they can also work together.
Here's an example. Say you have a law firm that specializes in criminal defense. You don't want to use Facebook ads because your audience will be too broad. Instead, you use AdWords and target people who are searching for help - RIGHT NOW! They might visit your site, and they might visit a few others. They might not commit to hiring anyone at this point, and save the decision for later. But you've already paid for the click.
So what now?
Even though they have been targetted originally with AdWords, you can still tag them with Facebook. Now when they get on Facebook, only the people who visited your site through your AdWords campaign will be shown your ads on facebook. This means that you can laser fous Facebook ads based on Google AdWords data! AdWords does also offer the same service, so if you use BOTH retargetting platforms, you can double your effort to make sure that clients who were interested in your service continue to see your ads everywhere they go!