VegardAa,
With JFBConnect, you can easily add comments to other extensions in a few different ways. We have modules that could let you put the Comment box in the same module position. Additionally, you could edit the template from AdsManager and add our {JFBCComments} tag so that the Comment box appears automatically with the ad. I've never used AdsManager, so you may need to get some information from them on how to add code to their templates, but it's usually a pretty straight-forward process.
As for moderation, Facebook allows you to easily remove any comments that you don't want there. JFBConnect can also be configured to automatically email you whenever a new comment is posted, so that you can stay on top of it.
Hope that helps,
Alex