>Facebook began as a network for Harvard students, then island networks for other universities, and so on, and it only got merged into one monster network later on. Because of that history, it tends to be a social network for people to connect to school friends, family, work colleagues, etc. (i.e. people you've met in real life).

>Google+ has much different beginnings.

Google's real name policy pretty clearly indicates that Google+ is intended as an extension of your real-world identity (like Facebook) rather than a forum for alternate personas.

>Twitter seems to be doing quite well allowing pseudonyms.

Twitter positioned itself as primarily a public venue for alternate personas. Kind of like HN, or pretty much any forum. I do not know anyone who actively uses Twitter to talk to their real-life friends. They use Facebook for that.

>Same with Hacker News. You wrote "pseudonyms have no place in social networking" but what is Hacker News if not (in part) a social network?

Hacker News is a forum with voting. It doesn't track any sort of connection between users, and doesn't encourage sharing of personal lives. I don't consider it a social network at all.

>An anonymous comment isn't the same thing as a comment under a pseudonym. People tend to use the same pseudonym (or set of pseudonyms) everywhere they go: people can get a sense of who they are, what they believe. They can even use that pseudonym when not online (e.g. Mark Twain never had Internet access).

I understand that. I fully support pseudoynm-based discussion boards like HN (and blogging under pseudonyms), and I am appalled at people who want to enforce real name use everywhere. Those are useful, essential to free speech, etc.. But Google+ and Facebook are not trying to build pseudonym communities, they're trying to facilitate non-internet relationships online.

>"Real names" and pseudonyms are just handles. They're like variable names when programming. The computer doesn't care what you call the variables.

Actually, Facebook knows you by uid anyway. I never argued that we should care what people are known as to computers, but that I should see my friends' vacation photos /location checkins/status updates under the name I know them by, not soccaluv18273084.

>Why should you care whether someone goes by the name their parents gave them or something else? Shouldn't that be their choice?

Because in social networking (or, since you have a broad definition of social networking, the kind of social networks intended to extend your non-internet relationships) I want to call people by the same name we use in person. So do a lot of other people, hence Google and Facebook's policies.