Eventually all good usernames/ email addresses/ domain names will be taken. Especially with usernames, many are simply are not used anymore. So how will websites like hn cope when no new user can find a satisfactory handle. Will sites like twitter/fb and even Gmail be abandoned by droves in the years to come?
Gmai started with one user and has doubled thirty or so times since then, and can only double a few more times before everyone has an address. Surely the precise limit isn't very interesting when we already know the number of digits.