We have articles on our site dedicated to different methods of reach calculations: Page Reach: Page-Level Reporting Post Reach: Post-Level Reporting Paid Reach: Why Your Reporting May Miss the Complete Picture
Jon wrote an article outlining the process of sharing a Facebook pixel or advertising audience here.
The pixel is an implementation of Facebook-provided code on your website which enables tracking, optimization, and attribution within ads on Facebook-owned properties. The Facebook pixel is a common theme across many of the resources on our site. You can check out our pixel-related articles here.
Jon wrote an article outlining the differences between different click metrics here.
You can use Ads Manager to analyze and compare audience sizes between these platforms, though it requires a bit of a manual approach. We have an article outlining how to do this here.