In a short post, the Times Higher Education provides some statistics of how UKs top 10 universities (not the libraries!) make use of Twitter.
This may be a supplement to this article on the use of social media of the US’ top 100 academic libraries:
Frank Boateng, Yan Quan Liu, (2014) “Web 2.0 applications’ usage and trends in top US academic libraries”, Library Hi Tech, Vol. 32 Iss: 1, pp.120 - 138, http://dx.doi.org/10.1108/LHT-07-2013-0093