I'd very much like to see official statistics on this question. Personally, I'd be very surprised if the majority of campaigns are primarily relying on D&D Beyond at this point, even though Wizards clearly would like for everyone to use it eventually.
That said, there's an important difference between having an account on D&D Beyond, and being an active user of D&D Beyond. A difference I don't expect Wizards to distinguish in any official numbers, which will surely focus on user counts rather than engagement. (Especially since Wizards is under investor pressure to justify the purchase of DDB, based on the investor call a while back.)