Good question...answer will depend on how you mean "user base"; I
imagine there's a wide range of people out there, from casual
hobbyists and others who tried it out, used it to greater or lesser
degree, and who still have copies on their machines that they fire up
from time to time...to people who are still using it fairly
regularly...to those who can't imagine programming any other way.
(Textual languages? Feh!)
Seriously though, the Open Prograph Initiative mailing list has a
subscription of about 70 people, of whom about only 35 to 40 seem to
be actively following the list. Many still use Prograph on a daily
basis. (BTW, the Windoze version evidently still works--as well as any
app can in that environment; while the Mac version will run and can be
use in Classic on OS X, it is not exactly stable. The mandate of the
OPI is to attempt to update the IDE and compiler, but Pictorius
refuses to release the source code....)
There are hopefully a lot of people still using it (or still
interested) who are not in the group, but I couldn't even hazard a
guess at how many.
So, in final answer: Very small.
--Ron