While Tumblr was talking about implementing ActivityPub they also started an option to 'blaze' a post - pay and it will be shown to more Tumblr users with a 'sponsored' tag.
$10 (estimated 2,500 impressions)
$25 (estimated 7,000 impressions)
$65 (estimated 20,000 impressions)
$150 (estimated 50,000 impressions)
and so it goes.