What distinguishes growth hacking from regular marketing? It seems that good marketing is impossible without delivering knowledge about your product to right audience.

There are cases when initial product is used to advertise or refer to other popular products which in turn creates popularity initial product. There is a famous Airbnb and Craigslist case when Airbnb offered users a share link to Craigslist which in turn brought them lots of users. Are such cases mostly legal and is it safe to refer to other products without collaboration?

How to use growth hacks when everybody else uses them and in order to create something cool you need lots of resources in terms of money or expertise?