I'm having trouble figuring out which would be a bigger accomplishment -- 1 billion on Facebook's platform or Google's. I'd imagine the average Facebook user (viewing/uploading photos and video, multimedia ads, pages with a zillion pieces of dynamic content, etc.) has heavier responses, but Google needs to do more legwork to put together the comparatively light response to a regular old web search.

Not to be pedantic, but this isn't really about usage so much as unique humans. I think the fact that 1B separate human beings utilized the service is more astounding than, say, number of page views or requests.