When your contacts share your Facebook Ads, that's a great thing. Unfortunately, it's not great for tracking.
When Facebook Ads are shared, they are no longer treated as Ads. This means that any tracking parameters on the Ad, no longer get added to the clicks.
What does this mean for you?
This means that if you have ads that get shared a lot, it's possible that you are getting leads and/or sales from Facebook, which won't be attributed back to Facebook. As you know, if the user's click does not have the Facebook tracking parameters in the URL when it hits your landing page, there is no way for us to know that a click should be attributed to a Facebook Ad.
So, if you feel that your lead and/or sales attribution is low for Facebook, shared ads could be a cause.