I've just set up a new site (call it foobar.stackexchange.com), before I realised that foobar.com was taken.
I can obviously set up another domain (fizzbuzz.com) and set up an alias, but would this cause any problems or confusion to users? Is foobar.stackexchange.com even visible to search engines once an alias is set up, or would all traffic go through fizzbuzz.com?