By Gerry Shih SAN FRANCISCO (Reuters) – Snapchat, the red-hot private messaging service, said on Thursday that it knew for months about a security loophole that allowed hackers this week to harvest millions of phone numbers and announced changes to its systems. An anonymous group called Snapchat DB posted the usernames and phone numbers of 4.6 million Snapchat users on New Year’s Eve, days after the startup – headed by 23-year old founder Evan Spiegel – brushed off warnings that its app still contained security loopholes. The hacker group, which claimed to be based in the United States and Europe, made the entire database available for download but redacted the last two digits of every phone number.