Apple PR: "1 Million Downloads on Mac App Store!"
Twitter PR: "900,000 Downloads of Twitter for Mac in 24 Hours!"
https://twitter.com/shawnblanc/status/23391070813425666
It's a joke, but I bet there is a heavy skew towards free software and new software. You're probably not going to pay for iWork a second time. It isn't all Twitter though; the guys at Evernote saw new sign ups from Mac increase by 1,800% (https://twitter.com/evernote/status/23116726547775488).