Without obviously knowing the details, it sounds like you didn't have a lot of friends in common, and you weren't really in touch with his family or the people who would have reached out and spread the sad news...?

Having recently had a death in the family, I can say that there were some people who we wanted to notify, people who certainly had known him long enough and well enough to deserve that, but whom we ourselves weren't in contact with and had no other contact details for than outdated email addresses. So very possibly they'll have a nasty shock like yours at some point... it sucks, but there it is.

It's a horrible situation and no doubt making your grief even more painful than it would normally be, but it doesn't sound like you should blame yourself for anything worse than just losing touch for a while for a friend, as I'm sure we've all experienced.