[koopajah]

Each twitter account comes with a unique numeric identifier in addition to your handle. If it didn't you wouldn't be able to login with twitter on some websites when you change your handle.

https://dev.twitter.com/docs/platform-objects/users

EDIT: After rereading your comment you might have been asking if it was possible to retrieve this id with a simple API call and not with OAUTH. It is possible with "users/show" as explained here: https://dev.twitter.com/docs/api/1/get/users/show

[lukashed]

Thanks, good to know. Though in the current version of the API (1.1), there is a rate limitation of 180 requests/15m (https://dev.twitter.com/docs/api/1.1/get/users/show), which could become a problem quite quickly on a website of that scale.

[ceejayoz]

The rate limit is done against the user's access token, not the server itself. A billion users gets you 180 billion requests in 15 min.