The huge drop matches the industry expectation before the Times instituted the paywall that traffic would fall off by 90%, which is the standard experience when a site moves to a paid-access model instead of free access.
“Times loses almost 90% of online readership” – Less than three weeks after the Times paywall went up, data shows a massive decline in web traffic. [via]
Although 90% is a shocking number (I had thought it would be worse), an online publication can at least project how much revenue they might make when they make the switch to a paywall.