aka "how to scare my family into stopping publishing their life online?"
I do not publish personal photos / opinions publicly online as a rule. I never gave hard thoughts about that but I believe that one should either explicitly put information to the world (typically professional data such as LinkedIn), or do not put anything. Again, this is a personal subjective opinion.
My (extended) family does publish stuff online, though. This is not obviously dangerous information (nudes, drunken parties), nor strong political/philosophical/religious/ethical standpoints. What they put online falls roughly in the categories of
- we are on vacation, look how happy we are (1)
- look at this cat (2)
- my children are cute (3)
The risks I can imagine for the various categories are
(1): "we are not at home, burglars welcome".
This assumes that their address is known (this is not a directly available information, though possibly available through reverse-engineering of some photos which may have some recognizable landscapes, or maybe GPS information) - but not pleaserobme.com-level known.
The fact that there is nobody home is also visible through casual observation of the house so it would be much easier to target them that way than to do some state-sponsored invigilation.
(2): no risk beside understandable social ostracism
(3): "paedophiles everywhere" or "they will track them down and abduct them"
This is a possibility but I am not sure that the fact that an image is online particularly increases the risk. Children go outside on their own, they can be observed, etc. Their children are also not particularly good looking or come form a wealthy family so there is no more risk on them than on somebody else.
Generally speaking, I am looking for elements (data points) which would show that online presence increases physical risks.
I am specifically not interested in digital tracking which brings in targeted advertising or similar soft threats - except if they bring in a physical components.