Facebook general counsel Colin Stretch said the investigation analysed 3,000 reviewer decisions following allegations that conservative issues were being suppressed.
The Trending Topics feed currently works as a mixture of AI and human input, with potential subjects being suggested via algorithm and then reviewed by staff.
They are a mixture of popular subjects discussed on the social network and sourced from 1,000 media organisations.
Topics with sources in foreign languages may also not be included on the grounds that the team may be unable to identify them, Mr Stretch explained.
The report did find that historically some topics that were discussed over a long period of time did not show up algorithmically.
Some Facebook users have queried the relevance of Trending Topics, which at the time of writing range from a "reported wardrobe malfunction" by US socialite Paris Hilton to the results of the Austrian presidential election.