|The number of Snapchat users by geographic location. Users are predominately located in New York, San Francisco and the surrounding greater New York and Bay Areas.|
Theoretically, if someone were able to upload a huge set of phone numbers, like every number in an area code, or every possible number in the U.S., they could create a database of the results and match usernames to phone numbers that way.
Adding that they had various "safeguards" in place to make it difficult to do that. However it seems likely that—despite being explicitly mentioned in the initial report four months previously—none of these safeguards included rate limiting requests to their server, because someone seems to have taken them up on their offer.