Of course you're spot on that traffic to the Site, in the form of prospective employees, is just as important. But there's no agreed-upon, industry-wide KPI to express this and i suspect that might discourage public disclosure.
as you can imagine there are maybe a dozen different ways to express site traffic. So for instance, unique visitors per day gives you an optimistic figure but without any indication of traffic quality (ie, how many searches, how many jobs they clicked, and how many they applied for); and of course at the other end is something like "unique applicants per day".
and again, on the other hand, paying customers (employers) while not the whole story, is a metric subject to less reporting variance.