FB should be more difficult as well because they aren't just showing streaming information. They're making (supposedly) intelligent decisions on what information to show you. Twitter is just showing you a "dumb" stream.
The killer app of Facebook - poking - does need to be reliable ;-)
Poking, while needing to be reliable is a 1:1 activity, so light on writes and easy to read, whereas broadcasting is difficult because of the wide variance in update frequency and number of followers from user to user.