[Ted S]

When you have a mid or large sized site making ad revenue tends to be fairly straight forward. Depending on the demographic, the value of the visitors and the sales/ marketing ability of the owner(s) forums can bring in substantial CPMs and with their high traffic there tends to be a lot of opportunity for return. Companies often acquire forums at a mid-sized stage before they're really hit their peak earnings in order to make a substantial profit by bringing them into an established private ad network where they can drive rates up quickly.

AdSense can also be quite profitable when put in the right context.