Plenty of people have added their real names in addition to their usernames. But even if they don't, contact lists often have real names.
I'm not sure if that's how you use Snapchat, but personally (and I may be in the minority) I only add people I've either met, or who are in some capacity a celebrity. You can also typically see how close a friend someone is by how many connections they have to other friends in a user's network. Snap users (like Facebook users, I think?) also tend to mostly interact with people they're actually friends with. So at the very least I think the data Snap has is at least as high quality as FB's (and I'm not at all disappointed we're comparing more with Facebook than Twitter, as the latter has had a much more positive market story.)