If she is like the Christian "Satan" (she is not from her description), then her worshippers should be an extremely tiny amount of people
Despite the fact that discusions about real religion are forbidden here ...
I feel strong urge to say: "You would be surprised ..."
Even tho it would most likely not be that kind of "Satanists" you probably know.
In what aspect and why are people worshipping?
There is as many reason as there are worshippers ...
People who have been wronged or just think so.
People who lost everything and came to the conclusion that their gods had abandoned them.
People who expected her sister Selune to improve their lives without having to do anything, and it didn't happen, so they started supporting her on purpose.
Those and many others.
Forgotten Realms is in a way a terribly dangerous and scary place for anyone who isn't exactly a powerful hero at level 10+. : D
And unlike our world, where one (very strong) religion ruled in certain regions ... all the people who live there have and have to deal with those problems to their choice.
Either they can worship good gods and go one way ...
Or they become bitter, they start to hate everyone and everything, and when they can't go well anymore, at least they make the lives of the others bitter. : D
If you wanted an example from our world ... I would recommend Hinduism.
There are gods TOOOOOOOO MANY than to name them all ... but far from all of them are downright good (or bad as the matter of fact).
Among the better known is Shiva the destroyer ... while we in Europe would have a hard time understanding why anyone would worship a god whose purpose is to destroy the universe, in India he receives great respect, because in their faith destruction is simply part of the cycle of existence.
(This description is, of course, terribly simplified ... but it could serve as an example.)