You can find some statistics on the top 20 according to different criteria on Steemwhales

as we can see, a lot of the bigger accounts aren't individuals, but App accounts or Platform accounts (Poloniex is rich XS)

Their reputation scores are pretty indicative of them not being posters.

But of the real people, some are getting there !

@papa-pepper, for example, is worth approx. 58,000$ XO (DO follow him... he's amazing, and Saturday he starts a new season of Steem-pocalypse !!! Payout to the 2 winning teams was 400 SBD each this time !)

@Ned is approx. worth...Oh... 10,200,000$

so yeah... technically got a millionaire or ten.

If we go by SBD... then we've got the highest accumulation by @cyrano at 18,000 SBD (Technically, Poloniex has nearly a million SBD, but hasn't posted so we won't count them XP)

So it depends on how we interpret it.

@pbock's "value" is 337$ a little SBd, but no Steem.
I plan on rocketing that soon XP