Donations (the button right next to your profile button in the header) were keeping the forum alive in the first time.
Nowadays, most of the income the forum makes comes from the ad-auctions (forum ads shown between the first and second reply on each site).
One thing I wanted to know related to this is that is there a single owner of this forum ? Again, are some staff members being paid here as well. Also is there a way to apply for the staff positions ? Tried to search about this but could not find any link.
Theymos is the defacto 'owner' or 'boss' of the forum, and is at least responsible for the day to day running of it and the finances and paying taxes etc. All active staff are paid in bitcoins but there's no salary per se. You can't really apply for staff positions either but the best way to become one is by reporting a lot of posts with a decent accuracy and if theymos feels you're needed he'll ask you.