Despite the increase in the number of institutional repositories worldwide, most of them seem underpopulated. At the same time, scientists are apparently willing to share copies of their publications on academic social networking sites. This paper compares the availability of the scholarly output in the institutional repositories of 13 top Spanish universities and in ResearchGate (RG). Results show that just 11.1% of the articles published in 2014 by researchers at these universities were available in their institutional repository in the first quarter of 2016. However, most of the articles that were not available in institutional repositories (84.5%) were published in journals that allow the deposit of the article in some form. In contrast, 54.8% of the articles were available in full text on RG. When authors who had uploaded copies of their articles to RG but not to their institutional repository were asked about their reasons, most replies focused on two issues: ignorance about the existence or operation of the institutional repository and awareness of the advantages offered by RG.