There was a tool at Google called 'percent' that you could run that would tell you what percent of the employees had joined Google after you (some measure of gravitas was associated with people who had been there 'a long time'). It was not surprising that Google hired a lot of people, there were like 50 people in my 'noogler' class. But the number of people reported as working at Google did not go up at a similar rat…
It is very easy to tell exactly who leaves google. It has a list available to any employee which is auto-updated and often includes any farewell message the leaving person sent.