Perhaps medical records are more 'viscerally' private to people than conversations with friends? It's easier to forget that big brother is recording everything you say when most of it is amiable chatter, but when it comes to having, say, incurable butt-warts, people become a lot more reticent, if only out of a fear of embarrassment.

Personally, short of deleting my account, I stay as far away from Facebook as I can, and try to minimise the amount of data I share with other services, although some compromises are still necessary or at least sufficiently convenient.