Which one of the following is not a social networking site?

Khan Academy is a free online learning platform of video lessons and practice, not a social networking site.

YouTube is a video-sharing platform carrying social features such as comments and subscriptions.

Facebook is a leading social network for connecting with friends, pages and groups.

Twitter is a microblogging social network built around short public posts.

The others revolve around user connections and sharing, unlike Khan Academy's structured courses.

Khan Academy was founded by Salman Khan and offers lessons across many school subjects.