This is a perfect example of what has been said about the recent revelation that the NSA is collecting the metadata for all phone calls (calling number, called number, etc.): you can learn a lot about a person just by knowing who they've communicated with and how often, without having to know anything about the actual content of the communication. If your girlfriend sees that someone with a female name she's never heard of is your number one Snapchat contact, that's probably all she needs to know.
