Everyone should read this blog post and the 2 followups. It brings a lot of insight into the parts of Twitter that bring it down and it has nothing to do with the message pushing part of Twitter.

I have a Jabber bot running that is hooked into the entire public timeline and it continues to run even when the main website has gone down. I don't think you can directly compare Twitter's issues with issues of Meebo, AIM, GTalk, etc.