It seems that Code4Lib hasn't discussed this., though the news is 2 weeks old. It seems that there are libraries using social share tools from AddThis", a company that has been using a technology called "Canvas Fingerprinting" to track users.
In other words, it looks like libraries are giving away the user-privacy store.
For example, AddThis is used by my public library's Polaris catalog (BCCLS).
I'd be interested to learn how widespread this is.
Here's the article from ProPublica.
And a follow-on discussion from Princeton CITP
The research article:
Founder, Unglue.it https://unglue.it/