Nearly every post made here is thanks to danbooru's linked account. Nobody comments, favorites or makes forum posts.
Where do they get the money to keep this domain?

It's mostly alive because of the other image database that slayerduck runs, the more lewd one with a green background. (Please do not post any links of it here)

Ohh, so they get the money from there.
But I still don't understand why do they decide to keep this up. It's that nobody uses it.

We actually get quite a bit more traffic than you would think, I see a good number of people that upload images and a couple of other users that like to comment. It's certainly not busy but people still use this site.