Some of the answers come from a Q&A session with Joseph Smarr a technical lead on Google +.
"I helped design and build a lot of the circles model and sharing UI for Google+. I was recruited to Google to work on "getting social right" in early 2010."
Given Closure is part of the recently closed Google Labs I think it is one of the projects that we can assume will be saved.
For modern browsers human readable URLs are maintained using HTML5's History API even though the system is mostly Ajax based. Older browsers present the URLS raw.
For the backend BigTable, a database that runs on the Colossus distributed file system (GFS2), seems to do most of the work. BigTable isn't available outside of Google but you can make use of it as part of the Google App Engine. Google's Colossus distributed file system is essentially Google File System 2.
The missing Google technology in all of this is the Google Web Toolkit (GWT) which would have allowed them to build Google + using nothing but Java.
"Nothing against GWT, but the engineers who started building Google+ didn't use it, and in general projects end up all-GWT or no-GWT, and this was the latter."
The Q&A session reveals an overall move toward integrating Google + with other Google systems such as Reader and so on.
When asked about the much-missed API the answer was non-committal.
"We're honestly still figuring out exactly what form our platform/APIs should take, but we're eager to hear input from would-be developers!"
I'm a technical lead on the Google+ team. Ask me anything.
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