100% of Facebook's value is real names.
That is it. There are hundreds of other services for instant/asynchronous messaging, photo sharing, etc. but only Facebook knows everyone in my life by the same names I do.
I can discover and communicate with anyone I might have heard about in my real life on Facebook because (and only because) they use their real name on Facebook.
Discoverability by real name is the point of Facebook. There are certainly other social networks (i.e. Tumblr, instagram, HN) that people use as online personas separate from their real identities, but Google pretty clearly wanted a piece of Facebook's pie. Pseudonymous Google+ would have been an entirely different community. Maybe a more successful one, but not in the same space as Facebook at all, and not what Google was trying to do.
Interestingly, people use Tumblr as a much more intimate venue, in part because you just can't hold that many people's pseudonyms in your head - at my school, only fairly close friends got to know each other's Tumblr URLs.