4.6 million Snapchat users saw their account information compromised on Tuesday night (via KMPH), with usernames and phone numbers posted for all the world to see. The only "good news" was that the last two digits of the phone number were withheld by the group that leaked the information.
The first tool, GS Lookup - Snapchat, comes via the well-known Gibson Security firm. There, you enter your username. If your account info was not compromised, you'll see a "You're Safe" message. If, on the other hand, your data was leaked, you'll see your phone number (minus the last two digits) displayed on the Web page.
A second tool, Snapcheck.org, works much the same way, except you can enter either your username or phone number. After submitting the info, you'll see be told whether or not your information was included by the hackers.
Notably, if your data was included in the leak, there's not much you can do about it, save perhaps opening a new account. Notably, if your data was included in the leak, there's not much you can do about it, save perhaps opening a new account. Sadly, that doesn't erase your phone number from the already circulating database, although that was not fully published to the now-downed website the hackers leaked the information to.
Your only complete out would be to change your number, something folks don't like to do even if they move. Don't worry about your provider giving you a hassle; point them to the details of the breach and they will surely help.
In addition, it's always good in this sort of breach to change your password. While that information wasn't included in the published database, certainly, if you are one of the many who tend to use passwords like "12345" -- or even if you are not -- you should change it.