I really do not know how Tumblr works but at I have an idea with how Instagram works. Create an Instagram account, invite your current Tumblr followers to follow you on IG. The promo post should not be one off but consistently at intervals. With Instagram, you can either decide to be niche specific or go broad like Celebrities, Trends, Fashion, Gossip, jokes, love quotes, Motivation quotes, romance etc. Build your following. With time, people will start contacting you for shoutouts or adverts.
When you go niche specific, you will get targeted audience and products related to the niche can be promoted directly by you as an affiliate or other product vendors. But when the niche is not specific like Jokes, funny, different ranges of products can be offered.
Also with hashtags, on Instagram, you will get almost immediate attraction to your profile or posts.
Hope this helps