Journal articles. In most cases articles are linked to free open resources. A few are only available via George Fox login.
Junco, R., Heiberger, G., & Loken, E. (2011). The effect of Twitter on college student engagement and grades. Journal of Computer Assisted Learning, 27(2), 119-132. doi:10.1111/j.1365-2729.2010.00387.x (Open access link to article.)
The following link contains the top tweets from an Educause conference that took place in February 2015. Those who were unable to attend the conference can benefit from viewing the tweets by those in attendance: Educause Learning Initiative (ELI) conference in February 2015.