I have been encouraged to try out Nextdoor, and I am convinced they could provide a good service. But, I cannot convince myself it will be profitable and stick

I have been encouraged to try out Nextdoor, and I am convinced they could provide a good service. But, I cannot convince myself it will be profitable and stick around. How will nextdoor make money? The normal rule seems to be--if you can't tell what is being sold, you are. Is that the case here?