  By linking to external websites, Wikipedia can act as a gateway to the Web.
To date, however, little is known about the amount of traffic generated by
Wikipedia's external links. We fill this gap in a detailed analysis of usage
logs gathered from Wikipedia users' client devices. Our analysis proceeds in
three steps: First, we quantify the level of engagement with external links,
finding that, in one month, English Wikipedia generated 43M clicks to external
websites, in roughly even parts via links in infoboxes, cited references, and
article bodies. Official links listed in infoboxes have by far the highest
click-through rate (CTR), 2.47% on average. In particular, official links
associated with articles about businesses, educational institutions, and
websites have the highest CTR, whereas official links associated with articles
about geographical content, television, and music have the lowest CTR. Second,
we investigate patterns of engagement with external links, finding that
Wikipedia frequently serves as a stepping stone between search engines and
third-party websites, effectively fulfilling information needs that search
engines do not meet. Third, we quantify the hypothetical economic value of the
clicks received by external websites from English Wikipedia, by estimating that
the respective website owners would need to pay a total of $7--13 million per
month to obtain the same volume of traffic via sponsored search. Overall, these
findings shed light on Wikipedia's role not only as an important source of
information, but also as a high-traffic gateway to the broader Web ecosystem.
