  arXiv.org mediates contact with the literature for entire scholarly
communities, both through provision of archival access and through daily email
and web announcements of new materials, potentially many screenlengths long. We
confirm and extend a surprising correlation between article position in these
initial announcements, ordered by submission time, and later citation impact,
due primarily to intentional "self-promotion" on the part of authors. A pure
"visibility" effect was also present: the subset of articles accidentally in
early positions fared measurably better in the long-term citation record than
those lower down. Astrophysics articles announced in position 1, for example,
overall received a median number of citations 83\% higher, while those there
accidentally had a 44\% visibility boost. For two large subcommunities of
theoretical high energy physics, hep-th and hep-ph articles announced in
position 1 had median numbers of citations 50\% and 100\% larger than for
positions 5--15, and the subsets there accidentally had visibility boosts of
38\% and 71\%.
  We also consider the positional effects on early readership. The median
numbers of early full text downloads for astro-ph, hep-th, and hep-ph articles
announced in position 1 were 82\%, 61\%, and 58\% higher than for lower
positions, respectively, and those there accidentally had medians
visibility-boosted by 53\%, 44\%, and 46\%. Finally, we correlate a variety of
readership features with long-term citations, using machine learning methods,
thereby extending previous results on the predictive power of early readership
in a broader context. We conclude with some observations on impact metrics and
dangers of recommender mechanisms.
