Here is an interview with one of the Workflowy founders. It seems to have been from sometime in 2016, and states that they have reached one million registered users. Obviously not all of them are active, but from the interview it appears that the founders/company are doing okay. It's a small company that seems to be self-funded (so they hopefully won't implode due to internal pressure/VC goals.)

I do feel the app has achieved a decent level of traction in the marketplace. I took advantage of the 1-year free Pro offer and now use it every day, and anticipate paying for it going forward.

I like Workflowy, but have found Outlinely on Mac and iOS so powerful that I haven't the least incentive to use it (Workflowy, that is).

Outlinely is getting better, too. One of its best features, for me, is the ease with which you can move items within an outline around - a very simple solution involving a "thumb" on the right-hand margin.

And now you can also mark outlines as read-only.