Re: Glad the people they busted got busted, but this is a drop in the bucket
TFA mentions "roughly one million Bitcoin wallets". So, to a back-of-the-envelope level of accuracy, I would take that as a first estimation of the total number of site users.
I don't know how you infer that it was a small-fry operation, though. Those 337 could include most or all of the admin-level users and/or some of the bigger contributors.