Disqus has been a useful commenting system for blogs. There is new like button added to build communities. Now if you view any Disqus powered web page (like Mashable.com), the commenting system is also optimized for mobile (see screenshot below).
The reasons why I use Disqus
1. Threaded Comments
2. Easier Comment moderation
3. Sync/ Import / Export for Blogger/Wordpress
4. “Like” for posts / comments
5. Social sharing options, Community profiles
6. Automatic email subscription of comments
Disqus is also expected to release a Lite theme for comments which is supposed to load faster (for those who are concerned about the loading time of sites).