FindAGrave actually helped me put to rest a family mystery - but only because the FindAGrave piece of the puzzle fits with all the rest of the pieces of the puzzle. Any family member who might have know when my g-grandfather died is also long deceased. Texas didn't keep death records at the time he died. I'm on the east coast, so extensive in-person research is a "some day" proposition.
However, a review of the information posted on FindAGrave turned up very useful information. The name listed was only initials, but they were the right initials. The birth year listed fits with multiple census data. The death year not only fits with the decade in which he must have died, but fits with family stories that g-granddad died when granddad was very young. The memorial initially had no grave stone photo, but a very kind FindAGrave photo volunteer made the trek to the cemetery and posted a photo for me. At the head of the stone is the compass-and-square emblem of the Masons. G-granddad was known to have been a Mason.
Is this "proof" of his actual date of death? No. Sadly, documentary proof probably doesn't exist. But it's as close as we're going to get and it's very valuable if only for that. Even documentary "proof" is only as good as the documenter.
This is what genealogy is all about. Weaving the threads into a coherent whole.