I at the moment offer posting packages as a service myself for payments and I also use the user upgrades feature as well on my forum as well as using Google Adsense as well to monetize my forum and help with the upkeep. We also allow members to donate as well to a campaign where we state what features we would like to bring to the forum as well and that helps if people would just like to help out.
To the best of my knowledge, making use of Google Ad Sense is what most forum owners use to make money from their forums or websites. They rely on it when they cannot be able to sell ads space on their website. Some people make use of Infolinks to make money from ads.