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I prefer my ID's universally unique. I worry that globally unique ID's won't scale as we begin to colonize other planets. (yes, its a joke.)

Stating it is a joke kind of ruined the joke.

we don't use incrementing ids, but needed ids that increase over time (so that you can sort tweets by them). hence, snowflake: http://github.com/twitter/snowflake

You don't even need to go that far. For non-critical applications (i.e., your web app), you can randomly generate a small string, say 12 bytes, using base-62 characters (A-Za-z0-9) to serve as a probably unique user ID (with VERY high probability).

Heard of GUIDs? Make part of it a timestamp, and the chance of collisions goes down astronomically further.

In some cases, including a timestamp in an ID can be giving away information considered private. Sure, you could just hash the resultant ID, but then you're getting back to random digits anyway.