The first interaction (or first touch) attribution model is focused on the first known touchpoint in the customer journey.

With the first touch attribution model, 100% of the conversion is attributed to the first registered click.

So if we take the example above, then 100% of the conversion would be attributed to Facebook ad #1

Given that it ignores all other touchpoints, it should not be the only model being analyzed. However it is very useful for brand awareness campaigns or if you focus on channels your potential customers will interact with first.