Twitter didn't always use Snowflake -- that was introduced in November 2010. There was another, much simpler algorithm used before that which generated much smaller IDs (e.g. under 3e10).
Yea.. "auto increment" in mysql. it SUCKED in so many ways, primary of which was that it required a single mysql database to be the primary for all tweet writes. Every tweet just incremented a counter by 1. As we scaled up that server got overloaded rapidly and became a huge single point of failure. Snowflake was introduced to solve several problems at once. =)