Ha, I wrote a thread about this after I had a run in with a vet on a facebook page.
advice about when to give advice
I think it depends on you as an individual if you can cope with people answering you back (let's face it none of us like being told we are doing something wrong). Some people will take it on board and some people won't. I get very upset with confrontation and take things to heart so I very rarely say anything these days and I am chronically ill and just can't do with the stress. If I was healthy and well I would probably do more about it.
If they follow you on social media, you could try to lead by example in what you do and show with your own hamsters, hoping that those that are willing to change will.
If animal care is really bad I have reported cases to PETA or reported them on the appropriate socail media channels.