It's depend on what you promote. If your content is related to crypto, you can promote in crypto forums. I just suggest the most famous ones. If your content is related to something else, you can promote your content in related forums. For example when I created content about politics, I promoted in forums related to politics. There are a lot of forums on the web and we always find people there. Steem is a blockchain for all types of content creators, we welcome here everyone and that's the mass adoption.