Because different ads take multiple and varied paths, there is no central
clearinghouse for information on digital ad inventory. Therefore, in spite of
recent efforts by the major digital platforms to make data on advertising
available for researchers, there are still gaps in our knowledge of the broader
universe of digital advertising, and there are issues of comparability. Because
each of the newly available libraries was designed and built individually by each
social media company, making comparisons across platforms is difﬁcult at best.
At its most basic, it is impossible to compare all paid political advertising from
the platform libraries because only Facebook makes more than just election-
related advertising available. The universe of what should be included in the
broader set of political advertising, however, is a challenging deﬁnitional issue
that would not be guaranteed to be identical across libraries even if Google were
Social_Media_and_Democracy