"the measurable externalities of facebook are pretty minimal, considering it is _not_ the entire internet, and there are many other well known ways of commiting speech, and many ways of doing it anonymously."

First of all: How did you determine "the measurable externalities of facebook"?

Secondly: "Committing speech" without an audience/with a heavily limited audience is about as useful as not being able to say anything, though. See also "free speech zones". (No, I am not saying that that's quite the situation we are in with facebook (yet), but that freedom of speech is more complicated than "you can speak somewhere").

"For me the strong linkage that Facebook does with my identity (and between me and others) has great value. It's a public place more than a private one, and for private things I go elsewhere."

I do appreciate the difference between private and public places and the use of identifying people for certain purposes. What I disagree with is total surveillance of public places, and also the unconditional requirement of identification. If I go outside, I don't expect to be invisible - that does not mean that I am ok with having to always wear clearly visible ID and an officer following me and recording everything I do and say. The same applies on the internet.

"A big part of the value-add in FB is the general assumption that "you are talking to this specific person that you have met in real life" (say high school friends). Some identification server at the least and centralisation."

I don't think you absolutely need any centralization for that at all, a web of trust in principle should do, I'd think. But in any case, a central authority for that purpose certainly wouldn't need to be more than a directory server that provides a mapping between "real" and "online" identities - essentially a phone book. There is no reason why such a service should be able to read all your communication, to see how much you communicate with whom, to track which websites you visit and like, to collect photos of you, ... and all the other stuff that facebook does in addition. Also, such a service would not need to have any network effect, as it should be perfectly possible to operate multiple such services competing with each other without that being an obstacle to the user, if the actual communication is independent from the identification service.

"Genuinely curious as to any ideas you might have on this. Did you have anything in mind? Or is it just speculation."

A bit of both, I guess?

One part is "just" speculation, though with some grounding in reality: If you have a system with a strong network effect under the control of a single entity, that tends to lead to a (de facto) monopoly. And monopolies tend to not be particularly supportive of anything that might weaken their position, such as commoditization of their service, and also have rather limited motivation for innovating themselves. A historical precedent of this would be the telephone operators in many countries: In some places, you were forbidden from connecting your own modem to your phone line in order to dial up some BBS, because the operators themselves were offering some new "interactive digital services" or whatever that usually were less interesting and more expensive than the innovation of BBS networks. Or more recently, mobile providers have been keeping internet access excessively expensive in order to protect their SMS business, and lateron telling their customers that they couldn't use the internet access for instant messaging. And in any case, starting a new service directly on top of the telephone or mobile network is difficult. Compare all that to the internet: It's a commodity by its very design (though some telcos have been trying to reverse that recently, see net neutrality), and it's absolutely trivial to start a new service - anything that can use a TCP connection you can just do, no need to ask anyone for permission, no need to convince anyone it's a good idea, no need to change anything about the network to support your new service, just do it.

Long story short: Centralized services with a network effect tend to be an obstacle for innovation because they prevent competition.

As a specific example to think about: How easy would it be to put an encryption layer on top of facebook? How easy might it have been on an open, distributed system?

The other part is the fact that a lot of (public) social interaction nowadays happens on facebook, and anyone who values their privacy is effectively excluded from that. What effects does that have on democratic decisionmaking, for example?