Disqus is one of the leading comment systems. This add-on allows you to add a comment thread to every element, connection, loop, and map overview.
Installing the add-on
Create a free Disqus account by visiting https://disqus.com/profile/signup/
Choose the option "I want to install Disqus on my site"
Create a new site (create a site for each project you'd like to use Disqus with). Make sure to write down the shortname Disqus assigns for each site.
Go to the add-ons menu within the project settings.
Activate the Disqus add-on by adding the shortname you wrote down in Step 3.
Voila! You'll now see the Disqus comment box on each element, connection and loop profile (as well as the map overview).