If you want to report a problem about a certain profile, go to the profile page, and press the button (…), then ‘Report’ and follow the instructions.
If you want to report a problem with a Story, you need proof that it is fake or that the members(s) the Story is about is/are someone else. Open Stories and press the “…” button, then ‘Report a problem’.
If you want to report a problem with a Talk post or thread, press the link ‘More’ next to the post and select ‘Report a problem’.
If you want to report a problem because you found a bug on the site, or feel some textual content should be fixed, hit the Feedback button and follow the instructions.