Not about OERs but similar questions are of interest.
Investigation of SEO Rankings of Institutional Repositories There is a need “to investigate whether links [from popular social media services] are responsible for enhancing SEO rankings of re...
"The survey reports that two blogging platforms appear to be primarily responsible for providing the high ranking which may drive traffic to repositories."
The two blogging platforms in question are WordPress.com and Blogger. One question, I suppose, is how much traffic would the publications get if they were exposed directy through these platforms compared to being in the repository.