There’s no single solution when it comes to positioning your AdSense ads. The best place to position your AdSense ads is where the visitor’s attention is focused and your own experience after experimenting various positions.
Generally, you should follow these principles:
1) Above or below first paragraph
Above the fold is encouraged by google. This would make placing your ad unit around the first post to be quite effective.
2) Immediately after last paragraph
With ads relevant to the site’s content served via the AdSense program, ad unit placed immediately after last post will answer the visitor’s “what to do next” question.
3) Between paragraph
Placing a leaderboard ad unit between thread posts is quite effective for forums as well.