The question https://stackoverflow.com/q/11077121/251311 was asked by a new account and what's interesting is that by default SO includes facebook user id in the generated name.
Is there some valid reason for that? From my perspective it shares more information than person probably would want to share.
Wouldn't it be better to give name like newbie-<SO_user_id>?
That user name is (like Somnath said), most likely because that user signed into Stack Overflow for the first time using a Facebook login.
Stack Overflow sometimes takes a users Facebook profile picture as well for an example see my SO display picture..
I'm not sure if you were serious when you suggested newbie-<SO_USER_ID> as a default screen name for new users. I don't like it. It's one step closer to sanctioning calling all new users n00bs. It's not professional and I feel that it has no place in the SE network at all. The standard user_SO_USER_ID is the correct way to generate a user name - its too general to be offensive in any way.
With regard to making a persons Facebook ID "public" I have to disagree with you. no additional information is made available to you just by having someones Facebook ID. You are sill limited to the privacy settings setup by the user. Furthermore - just someones profile picture you can extract their Facebook ID -
//fbcdn-profile-a.akamaihd.net/hprofile-ak-snc4/48745_504135887_1363645060_q.jpg
The digits in bold are my Facebook ID and I'm not worried about making it public because I have a firm grasp on the Facebook privacy policies and no one will be exposed to material that I have not explicitly made public.
With regard to why SE decided to generate usernames by someones Facebook ID - I think you'd have to wait for an answer from a credible and/or official source. (I.E the dev team or an SE employee).
Remember that the automatically generated username is in no way unique. The user can change it at anytime.
Related - Change default username from "Unknown" to "User{number}"
(On this post they talk about specific login methods yielding different auto generated user names)
" So if there was not fbid in the name - we wouldn't know who he is." - It is totally possible to be anonymous here. We don't really have to know who someone is in the real world.