On most social networks, you have to make a username when you sign up. Not only that, that username has to be unique, no duplicates allowed. Facebook on the additional hand just takes your real name, no username, and it doesn’t matter if there’s a name by now on the site with the same name as yours.
There are probably hundreds of factors that add up to clarify 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.
We reckon there are three main reasons why using real names and not requiring usernames has helped Facebook grow larger than any additional social arrangement on the planet.
By removing the need to reckon of a unique username, Facebook has made it simpler to sign up. Just enter your actual name, there is no threshold where you have to reckon of a excellent username.
It also lessens the risk of initial frustration during the signup process. You won’t have to spend time thinking up a fantastic username only to have the site tell you, “sorry, that username is by now full.”
Which leads us to…
The larger the user base of a site becomes, the more of a problem usernames will become. It will be harder and harder to find excellent, unique usernames because the name space is too saturated. Compare it to the way many struggle to find a excellent .com field name for their site these days because so many variations are by now full.
Could this be one of the reasons Facebook has been able to grow to such an unprecedented size? Remember MySpace? It didn’t grow much past 100 million users. The latest number from Facebook is 350 million active users.
If Facebook enforced unique usernames it may never have grown into the hundreds of millions, because there would have been too much username overlap.
Then of course there’s the huge social thing in this area using your real name, connecting you with real public you know, keeping you in touch with friends and family tree. Real names and real public.

Facebook’s site interface works very hard to connect you with public you know and you’d be hard pushed to argue that this hasn’t made Facebook all the more addictive and more engaging. And this, of course, also helps the site grow.
Using real names could have worked hostile to Facebook if public had been reluctant to give this in rank to a website, but the truth turned out to be quite the opposite and made Facebook stand out from the rest of the social arrangement pack by being more personal in that sense.
Yes, we know what you’re thinking by now: “But Facebook has usernames!”
It does, but that’s ok because it’s just an extra feature added last summer mainly to give you an simple URL to share. It’s in no way mandatory. Straight from the horse’s mouth:
Choosing a username is optional and will give you a distinct Web address for your profile. It will not change the name that appears on your profile, in search or elsewhere on the site.
In summary, by using real names and avoiding (mandatory) usernames Facebook has:
This recipe won’t be right for all sites, and there will always be voices raised in this area privacy on sites where you enter your real in rank, but for the social networking aspect of Facebook it has clearly been ideal.

From:
Was Facebook’s utmost go to skip usernames?