The Disqus Comment System plugin allows people to comment via their Disqus account or Twitter/Facebook. It’s more conducive to visitor engagement, cuts down on spam, and looks great out of the box (although there are only two color schemes, light and dark, to choose from).
After activating the plugin, you can go to the Comments > Disqus panel to configure it. You will need to log into your Disqus account and add your URL as a new site on the Disqus website. Typically, you will want to export your comments from WordPress to Disqus. If you have a lot of comments (i.e. over 50), you might want to do it faster by exporting the comments from the Tools > Export menu, then upload that file to Disqus.