That’s actually part of another study that I plan to publish (soon, I hope, but don’t hold your breath). Anyway, as a preview, I have two charts showing membership numbers, both on a monthly basis and as a cumulative sum, through the end of May. You’ll see that after the rush to sign up in March and April, both registrations and subscriptions pretty much dried up in May (registrations did pick up a bit towards the end of the month, but I don’t know if that’s because of schools letting out for summer or not).
As far as numbers go, the SDMB homepage now shows 46,493 members, and the Forum Leaders page shows 3,896 subscribers.