A lot of people post very sensitive information on their Facebook profiles without realising that their profiles are public.
It's a simple task to correct your privacy settings, and as geeks we are more inclined to search for those settings and configure them, but the average user has no idea. Unlike Twitter, Facebook appears to be closed so most users assume that it is.
Is this irresponsible of Facebook? I don't know. But hopefully services like Openbook will help educate people.