yeah, if you say cheapest or cost effective way, yes i agree, but effective advertising? these i respectfully disagree with, except that YouTube channel has millions of subscribers and viewers that tune in daily to watch those videos, otherwise other means of marketing is required to draw people's attention to the project.
Separately, all types of advertising are ineffective, but when you start using them together, you get a very good result. So it's worth all kinds of advertising to use always.
Optherium had a lot of different advertising - articles, conferensies, videos, even TV. They have everything types of promotin. No they have bounty and airdrop.