The naive explanation is that they have "worker processes" running per service per user which either streams or polls content, but this seems like an expensive proposition (or is it?)

Are they using something else such as the Twitter "site-stream" functionality so that they can multiplex several users as part of one API call?

(Original Question posted on Quora: http://www.quora.com/How-do-sites-such-as-Friendfeed-and-Flipboard-scale-out-their-social-data-aggregators)