I'm very active on Quora and hadn't heard of it before.
151 contributors, all authors. 999 followers. My guess is, all authors.
The welcome post is inviting everyone to post, including readers, but there are zero readers on there, just Indie authors.
It's like everywhere else. All Indie authors looking for somewhere to advertise for free, and no actual readers.
Now, if it had a membership of tens of thousands, and half of them were readers, it might be useful. But 999, mainly authors, is pretty useless.
My guess is, they're trying to get authors in, when they should be getting readers in, which is going to be substantially harder.
I seriously doubt it will be of any use for promotion purposes.
For example, Mercedes Lackey has a following there of over 25,000, and her books debut about the same as mine do, so that list is worthless from a sales perspective.