Facebook's built-in conversion tracking does a really good job of sorting these things out. They run some complicated scripts to determine whether their ads was the origination of a conversion.

However, if you really want to do your own segmenting I'd work out a system to send users to a unique conversion page based on where they entered the site. It's actually pretty easy and and would give you a definite answer as to where your best / lowest-cost conversions come from.