Reddit is a topic based social network where user can post links, which can be upvoted and downvoted. Trending and controversial links can generate a lot of engagements: comments and shares. According to Buffer and Statista there are now 1.2 million topics, called subreddits, on Reddit.
A common misconception is to think Reddit as a marginal social network where only those special interests gathered. Clearly Alexa Ranking is saying otherwise.