How many redditors know other redditors (by their username)?

I know that many people are aware of friends who are redditors, for instance, but how many of you know other people in real life by their username. I see so many posts about people changing usernames once someone learns who they are.

I just think that reddit seems like a totally anonymous social community, which seems a little contradictory to me. I'm sure there are exceptions in various niche subreddits, but that is my general idea of this place I love so much.