ACLs (Access Control Lists) are the real benefit that social networks give. It's a system where loads of people already have an account, so it's easy to share things (e.g. photos) with a limited set of people. Obviously you can ask people to sign up for an account on <site where you photos are>, but asking people to sign up for Yet Another Web Account just seems 'dirty.' You could host it somewhere with a single password, but then it's hard to revoke access, or sent certain sets of photos to certain people. You could have per-album passwords, but now people have to remember (or write down) a list of passwords and map them to the albums that they are associated with.