We've all experienced twitter's spotty performance, but lately it seems contagious. Nick O'Neill wrote today about newcomer Plurk and its performance problems. Perhaps as O'Neill says reliability and uptime is not that important for a free and useful service. I think we've proven with twitter that a useful service, strong community and rich ecosystem of applications will keep people coming back again and again.
According to Pingdom's uptime overview, twitter's uptime peaked in March at 99.59% and has declined each month since. June to date performance is just 94.7%! Yet twitter is on a significant growth spike, approaching 2M users, according to Compete, up 14.7% in May. So maybe uptime doesn't matter, but it just gives us something to complain/kvetch about.