My exact thought. Good thing I Control + F'd "800" before duplicating your comment.
800 employees is especially insane for a serial entrepreneur like Stuart Butterfield who should know better. Then again, the Twitter guys including Ev Williams were serial entrepreneurs and yet the overhired too. It seems an interesting coincidence in the technology sector that viral user growth often can lead to viral hiring.
In my opinion, per-employee metrics (ie. revenue/profit per employee) are a great way to make sure that you don't overhire [1].
[1] http://www.businessinsider.com/revenue-per-employee-at-apple...