Generating random number as user identifier

I have been looking at the Unique ID (UID) generation for Aadhar (an Indian equivalent to Social Security Number)

They generate an 11 digit random number. I am also working on an application where I need to provide unique ID to users which needs to be valid an unique throughout there lifetime (expected number of users 40-60 billion).

When looking over the web for similar schemes, I found that simplest way is to generate each digit randomly and concatenate. So my questions are:

• Is generating each digit randomly correct way to generate such IDs
• Are there any established technique for the same ?
• What considerations one needs to take care of when designing such a scheme ?