Unlike Reddit or HN, Facebook is the worse social network for having any kind of meaningful conversation. The UI is terrible once you get past 1st level replies.

1) There are Facebook groups not only for cities, but sometimes even for individual neighborhoods within those cities. People gossip with their neighbours, complain about local politicians, or announce community events. These groups may represent a goldmine for the study of local history.

2) A lot of the travel community has abandoned standalone forum websites like Lonely Planet’s Thorn Tree or Horizons Unlimited for Facebook groups. The way people have traveled in the past is something that tends to interest later generations, as one can see from books that still get published about 19th-century round-the-world travelers or the Istanbul-Kathmandu trail of the ’60s and ’70s. Yet much of the detail of travel in the last decade or so risks being lost if Facebook goes.

Robert Caro's experience in uncovering the watershed moment in Lyndon B. Johnson's career might change your views.

"The Secrets of Lyndon Johnson’s Archives" https://www.newyorker.com/magazine/2019/01/28/the-secrets-of...

Any historian would disagree with you. The gossip and petty politics revealed in small-town newspapers, people’s correspondence, or diaries represents a resource they find valuable for understanding the history of a place. With the death of small-town newspapers, Facebook city and neighborhood groups have partly filled the slot that they used to represent. Yes, there is now Reddit, but there are several years where Facebook has a wealth of content for community events but Reddit had not yet grown large enough to cover them.