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It's free, yes. Unfortunately this isn't built for facebook marketeers. It actually filters your customer list by accept marketing status before applying segments which will lead to poor lookalike audiences because a lot of data is missing. For example if you have 5000 high value customers but only 3000 of them are on your email list, this app will only sync 3000 which means any lookalike you then build will be of poorer quality. The whole point of using an app is to capture the data that Facebook misses not to further reduce the data.
I'm guessing developers want to cross-sell their email app rather than taking time to build an audience builder that works. Fortunately there are a lot of alternatives on the app store.
ps. It upsets me to see apps like this mislead people who may not understand how audiences work. The description says "Facebook pixels don't pick up every customer" which is correct. But then it says "Our app captures all your order data and seamlessly syncs it to Facebook" which is completely incorrect. I have pointed this out to them and they admitted it's limited so hope they update the app description with something more accurate soon.