I just got back from a couple of friends from a small holiday gathering and I was really surprised that this has happened. It seems that some sort of caching issue occurred on the Steam store and users could see other random people’s account information.
For now I strongly advise not to use Steam at all and don’t buy anything because you could be compromising your data. I’m pretty sure they will fix it asap but it’s better to watch out for stuff like that. More info available here.