The data between GA and FB or for that matter GA and any other platform like Bing / Adroll etc will differ and in some cases differ substantially. The main reason for this is the fact that each of these platforms does attribution of conversions differently. Each of these ad platforms wants to take as much credit as possible so if a person clicks on an ad on any of these platforms and then purchases they will claim the conversion.
On the other hand, GA has an internal attribution model per which it gives credit of conversion to the last source used to visit the site before conversion. The model is called last non direct click [it excludes the source direct from conversion if it’s the last source and there is another source prior to it].
So as a result, where FB is claiming credit for all 12 people that clicked the Ad, GA is giving credit to FB when this was the last source and hence the difference. Same would likely be the case with Bing.
Ofcourse, deciding which of the two is the right model is a separate debate but in my experience, the truth lies somewhere between the two. By default, GA uses last non direct click model but under conversions >> multi channel funnels you can also see how much conversion credit sources have had for assisting traffic.
