The idea of a monthly service is not a bad one, but they need to offer something other than a thing everyone already uses for free. Tumblr already has other numerous one-time purchases that could be included in a monthly sub, like badges, Blaze, the crabs, etc. Getting one free Blaze per month with your sub, free crabs to give out, higher upload limits, stuff like that. A sub would need to leverage their existing features.