I asked the same question. The answer is that there are a bunch of instances (probably 15-20) which have thousands or tens of thousands of new accounts (<1 week old) but have barely dozens of posts. Here's a sheet made by @sunaurus showing the effect. A bunch of the explosion is in open signup (no email, no captcha, no verification) and there is zero interaction on the instance. Could we be seeing half a million lurkers on instances with <200 comments combined between them in the last couple of days? I suppose it's possible, but it seems unlikely.