I used to post my art on Instagram specifically but I deleted my account not to long ago, though I never saw it as something dangerous. I think your points need to be taken into consideration because art theft is something that exists and people need to be cautious about it. There are downsides to posting your art - your ideas can get stolen, and unless you have a big audience, there aren't many things you can do to prevent that from happening. However, at the same time there are many nice things about sharing your art on the internet! Probably the main reason I posted my art was to get feedback and improve. I'm not sure why I deleted my account though. Perhaps because I never got the exposure that I really wanted? Well, to answer your question, I have never really thought about the possibility of it being dangerous.Everything has it's own risks, though - so I believe the best thing is to just not stop and point out when art theft happens.