A friend of mine posted an article about this, and the two greatest threats the article mentioned were strangers (pedophiles) seeing the photo (so use privacy settings then!?) and friends/relatives downloading & re-sharing the photo (and then strangers seeing it). Which then led me to ask how sharing on social networks would be materially different than emailing to all the people you would share with. I absolutely see how there are differences, from a legal standpoint, from an expectations standpoint (social networks are built to facilitate sharing/virality where purple may be more thoughtful about email attachments). And there's the detail that although you can't use the "share" function and change the audience of a post/photo (on fb), you can always share a direct link to the image (which, from a standpoint of circumventing privacy settings, is similar to DL&repost, but easier).
Still, it seems to me that short of running your own password protected website from your own server using no commercial backup tools, any online sharing is putting your photos'/data privacy in the hands of the company(ies) whose product(s) you're using, and your audience. So it's worth comparing specifically what risks an alternative sharing strategy protects against (vs assuming that emailing or dropboxing your photos is safer).
Personally, given how sensitive the topic is, I don't post other people's kids without express permission (even if they post their own kids, they may want to maintain control of the audience), though I have no qualms showing them off in person (since that doesn't cede control of access).
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