Third-party commenting engines like Disqus, Facebook Comments or Intense Debate make it easy for you to enable commenting on any website /blog but there’s one downside – some of these commenting engines are implemented in JavaScript and hence search engines may not be able to read /index the comments that visitors are writing on your web pages.
In other words, your web pages will rarely get any SEO boost as comments are rendered in JavaScript within an <IFRAME> and hence the text is not visible to search engines.