100,000,000 users, many with upwards of 1000 friends, with no obvious way to determine the most frequent?
You could sprite the top 100 users, but that wouldn't have much benefit to most people. Top 1000 is likely too big to send out to every user.
Just sending them as we do now is lots of HTTP requests. If we could somehow reduce the requests... Ah, pipelining, or even better the built in feature of SPDY.
In reality right now, Google+ uses SPDY for everything but the avatars, which are served over normal HTTPS from https://lh3.googleusercontent.com/. Wonder why?
I just opened my G+ page and count 50+ user thumbnails. Many modern social websites have lots and lots of small unique user-specific images.