Apart from things like where you live and the pages you follow, it primarily looks for two broader signals - Topics that are being mentioned a lot and topics that suddenly seen an increase in mentions, Re/Code reported.
For example, singer Justin Bieber is mentioned often on Facebook. So the total volume of mentions is always high and is not a good indicator of whether or not he is part of a trending topic.
So, Facebook would look for a hike in mentions relative to the normal prattle around Bieber.
"This means that things that trend are not just the most highly mentioned people or topics, they have to be tied to some kind of relevant event," the report said.
Once a topic is identified as trending, it is approved by a human controller, who also writes a short description for the story.
These people don't get to pick what Facebook adds to the trending section.
"That's done automatically by the algorithm. They just get to pick the headline," the report added.
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