Perhaps of interest: the US Congressional Research Service, which is a non-partisan research group within the Library of Congress, issued a short report on the current state of defining and regulating online platforms: https://crsreports.congress.gov/product/pdf/R/R47662
This is very US centric and focused on potential congressional legislation, because the audience is meant to be US Congress members. But it could be a nice primer/overview on current concerns.
cheers, Phoebe