As the previous attacks was reaching peak activity, Twitter went offline.
The guys at Twitter are yet unsure of the cause of the problem (http://status.twitter.com/post/157160617/site-is-down).
Most likely the Koobface attack and Twitter going down at the same time is just a coincidence - but hey, there's a good part about this story too: at least we're not seeing new malicious tweets anymore.
UPDATE:Seems Twitter has changed its IP address during the downtime we're currently seeing. The server is responsive to pings, but not HTTP requests. Signs of a DDoS attack?
UPDATE 2: Twitter has confirmed it is "fighting against" a denial-of-service attack.