While people will not like you monetizing their posts, it is ultimatly something that only runs on your website. Of course if you federate out the posts people make on your website, then people will be able to access them on other sites without seeing the ads.

Injecting ads in the posts themselves, and federating them out to other servers however would be a different matter and likely get your server defederated relatively quickly, especially if you try to track impressions through embedding images or so.