The AdSense ads are contextual based. AdSense have difficulties in serving relevant ads in a forum.
Most forums deal in things like entertainment and games.
So the ads served may be ads, related to, wallpaper, etc.
Forums do get a large number of visitors everyday,
But then, getting a large number of visitors daily is of no use,
Even if it is a very popular forum, if the visitors have no intention of clicking ads, simply because they are not relevant or/and ads that aren’t attractive to click on.
All these do not make AdSense worthwhile to paste on forums. One can therefore say that, 80% of the time AdSense does not perform well for the publisher in terms of revenue in a forum.
A forum which is purely based on Q&A, where people are asked questions such as
“Can anyone recommend a lodge in Africa?” could do a bit better.
However it is still better to go in for a blog or site if you as publisher want to make a reasonable amount from AdSense. But also remember that everything you do should not be about tghe money.

ADSENSE IN FORUMS