WordPress.com added a new feature, that will allows users to post comments on WordPress.com blogs using their Facebook and Twitter account. This advanced commenting feature on WordPress.com Blogs, give users to ability to users to choose which identity they want to use for commenting on WordPress.com blogs. Users can use their Facebook, Twitter and WordPress.com account to post comment on WordPress.com blogs. Apart from this you can logged into multiple services means you can stay logged in to Facebook for convenience, but still leave a comment through Twitter or your WordPress.com account.
This enhanced commenting feature adds an extra layer of “real human identity” to the comments section of your WordPress.com. Most real people want to post comments as quickly as possible and they will go for some quicker way for posting comments on WordPress.com blog. So it is most likely that they will go for Facebook or Twitter accounts. If you don’t want to use your real identity, than also you can post comments on WordPress.com blogs as a guest using your email Address.
Facebook and Twitter account information is just used for authentication only. The comments posted on WordPress.com blogs using Facebook and Twitter accounts are not posted on Facebook and Twitter, however WordPress.com is working to have a better integration with Facebook and Twitter.
Currently this feature is not available for blogs using self hosted version of WordPress.