Not so much into stats but isn't a good statistical sampling has to have a uniform distribution over population space like strata/demographics? I wonder what t

Not so much into stats but isn't a good statistical sampling has to have a uniform distribution over population space like strata/demographics? I wonder what their sample size is for 1.56 billion internet users (http://www.wolframalpha.com/input/?i=total+internet+users http://www.wolframalpha.com/input/?i=total+internet+users). As a side note: Alexa is an Amazon.com company!, when did that happen?