For advanced digital marketers, the next step of course is looking beyond last click data. The attribution model you view your data through plays just as large a role in determining the success of it as many of the other factors that this article and others focus on.
Since you have conversion tracking setup for FB, FB conversions will obviously be higher than what GA shows in its standard reporting since FB's own conversion data is the equivalent of awarding 100% attribution to all assisted click conversions.
What would be more interesting is to see how this stacks up along with your other channels with various attribution models.
For those that are wondering if FB is working for you and looking at last click conversion metrics, you're doing it wrong. For a large % of scenarios, Facebook tends to play a much bigger role at the beginning of the conversion funnel, ie. the "Awareness" stage. This is also true of many forms of display advertising. To expect it to perform strongly on a last click model is unwise. First click models are likewise not great, but even that stark comparison will give you a sense of the contribution, and the GA multi-channel reports can show you that overlap.
I'm constantly blown away by how many amazing attribution tools Google gives away in the free version of GA and I'm always disappointed that none of the marketing articles that hit HN focus on that piece of the puzzle.