What kind of data are we talking about here? I'm sure there are all kinds of metrics and analytics available but I'm interested: What are the "rawest" forms

What kind of data are we talking about here? I'm sure there are all kinds of metrics and analytics available but I'm interested: What are the "rawest" forms of user interaction that Facebook makes available to these companies? I'm not in the business of using this data and neither I suspect are most of the Guardian readership so it would be nice to see this elucidated.