Well, the breach was back in February this year, so I hope they have put better security in place since then. I've seen ads for Weebly all over the place, but never realised they had that many users. Good luck with your application... :)
accounts doesn't mean the same thing as current or active users. a company very likely doesn't delete accounts right away or at all, even if the service has been cancelled. for example, a few years ago microsoft's live ID system had well over a billion accounts. but MAU was only around 450M. and that is with culling / deleting accounts after a year if they were unused.
While I agree the two are different, I also think in weeks case accounts could translate into multiple users. I know for a fact that my wife and her sister both share a weebly account for their business and I believe they have to share a login to do this.