I think that it is the only major leading source from where we can actually earn bitcoin
but i am not sure about trading
Certainly, trading is the major source of the bitcoin. As you can see, most of investors invest in bitcoin to trade it, and used it as transaction. The reason why there are campaigns is to promote some gambling site. Those gambling site helps the investors to have an entertainment and to take a chance to earn money from their luck.
As quoted trading is the best option for earning good with bitcoin, campaigns are not reliable. If we get slot we will be paid till the campaign lasts. These days due to the lots of earning availability campaign were given importance by small user groups.