On most social networks, you have to create a username when you sign up. Not only that, that username has to be unique, no duplicates allowed. Facebook on the other hand just takes your real name, no username, and it doesn’t matter if there’s someone already on the site with the same name as yours.
There are probably hundreds of factors that add up to explain Facebook’s success, but the question is if using real names instead of usernames isn’t one of the key features that have helped Facebook grow as large as it has.
Great article, worth a read.