You can see in the Chrome store how many active users the plugin has (which is only one part of what rbutr does): https://chrome.google.com/webstore/detail/rbutr/ocnieghejikn...

So it says about 10,300 active plugin users.

A more appropriate plugin for the OP's question is Hypothes.is. They're open source, non-profit and have raised over $1.5 million. They have about 7,300 users according to the chrome webstore: https://chrome.google.com/webstore/detail/hypothesis-web-pdf...