If you heard about the NCIX story where they basically abandoned their servers filled with users data (over 13 years of data) and someone scooped them up and tried to resell them on the black market, one could think that a similar fate is possible.
source : https://www.privacyfly.com/articles/ncix_breach/
Obviously if Facebook was going under it would probably trigger a huge legal process on how to handle the data but it clearly doesn't happen for smaller businesses...
Interestingly, Facebook owns your data. I believe if they wanted to, they could close the company tomorrow and put a facebook.tar.xz of everything they collected on archive.org or somewhere else.