Sundar Pichai gave some numbers during the I/O keynote, saying that 67 of the top 100 startups, 58% of the Fortune 500 and 72 of the top 100 universities have "gone Google": https://www.youtube.com/watch?feature=player_detailpage&v=wt...

I'm not sure what that means exactly and the numbers certainly encompass more than companies using Drive as a replacement of Office, but I found them suprisingly high. He also mentions a bit before that Drive now has 190 millions 30-day active users.