What's the practice to generate long user id like those on Facebook? Now I am about to launch a site and I have trouble coming up with a way to create long user id(e.g. 4000002). 

Do I set id to auto-increment(Starts at 0) and then add something(e.g. 4000000) to this id and put this new number in a new column in my table?

Does any body have a better practice to achieve this?

Thanks in advance!
Why not just set the auto_increment to 4000000 and let it count from there?

ALTER TABLE `table` AUTO_INCREMENT=4000000;


Also, make sure your id column can fit large numbers (bigint or similar)