I bet most of us here at bloggers.com has a facebook account, am I right. I think there are more people using facebook regularly than gmail and having a facebook comment box will surely boosts your blog traffic.
Here are some of the advantages
More convenient for visitors. Most people while surfing the web has their facebook account opened, having this comment box will not bother them signing up or entering captcha's symbols when they want to comment.
It boosts traffic. Visitors has the option to post their comments on their profile and when someone on their profile liked it, it will appear also on their profile page and so on.
Threaded comments. Some blogs has no threaded comments on their default blogger comment box. Using this plugin will increase conversation from your daily visitors.
No spam comments.
If you want to install this plugin on your blog just visit this link :-)