Landing Page Performance report provides data on visits to your Marketo-hosted Landing Pages. The social channels column you see in the report, if you hover over them it says "Visits originating from Facebook, Visits originating from Twitter, and so on..."

What I understand is, the data populates in the column when the landing page is shared on these social platforms and people click on the link to open the page. Hence, a visit is counted from that particular platform.

Also, here are few other Marketo docs that might help:

Basic Reporting - Marketo Docs - Product Documentation

Add a Social Button to a Free-form Landing Page - Marketo Docs - Product Documentation

View Social Performance - Marketo Docs - Product Documentation