As covered in a previous writeup: Installing, configuring, and integrating isso (commenting web app) into Confluence, I mentioned that I would cover integrating disqus into confluence.
Now before you go ahead and integrate disqus with confluence, be aware that there are other commenting (some of which are open-source) that will likely fit your needs better - especially if you care about privacy.
In any case, your free to choose your poison (as it were)...
Integrating into Confluence
The below was modified to suit my needs from David Simpson's nice writeup on integrateing disqus into confluence (from back in 2013).
First let's get there. Click on the cog icon in the top right of confluence and click General configuration:
Click thebutton and paste (and edit as required) the following:
You'll need to replace
in the code above with your own disqus address.