chill down folks. as for myself, i am in fact 50% dependent on google traffic since i am in the game for several years and get about 50% repeat visitors apart from google. when i started, it was nearly 100% dependency.
and i am 100% dependent on adsense income, aside from selling some ad space directly.
you know what? that doesn't make me nervous. i'm living good from that and i'm saving money every month for the worst case. i can sleep well at night, no problem.
in my area at least 85% of all search engine traffic comes from google, so it is absolutely senseful to concentrate on this revenue generator. traffic from other engines is peanuts.
every online business has different conditions, so beware of generalizations.