I spent a little while trying to implement a twitter back end for fun during their fail whale period and it's really just a data structure problem. (Edit: Figuring out which of the 240 million accounts they should be sent to.) You can literally get the basic freed process running 10k tweets per second with 100 million accounts on a single PC and a 1Gbit Ethernet connection where the Ethernet connection is the bottleneck. Fanning that out to various places to listen in is a fairly basic scaling issue.

Not that I took it vary far, but my first stab to see what the scaling issues would be was actually plenty fast to run there feed process at the time.

PS: The 'trick' is to keep two lists one everything a user follows, and another is everything that follows each account. For showing the messages to someone when they log in you keep the last 10 message Id's with timestamp or sequence ID so if someone follows 5k accounts you can avoid looking at the vast majority of those messages then sign them up the device to listen to new messages. (Sure, sometimes you will need to look past that top 10 but it's rather effective.)

(And for the downvoters I can upload some code if you want to see it.)