Hm... a while back i ran a test. Along my authority site I launched a porn tumblr. The test was to check out what would be more sucessful. A plain porn tumblr or the authority site I put a lot of effort into.
Well it turned out that for the first 6 months the porn tumblr outdid my authority site by a hundred fold.
but now here is the interesting part: I did get a seo optimized theme for my tumblr and further tweaked some stuff (most things that are "seo optimized" actually aren't *lol*).
Anyway the interesting part is: I did not buy an expired tumblr but started from scratch and only targeted one single keyword (has about 7 mio direct entries on google...so talking about competition)
While i only managed to get into the top 50 with my main keyword, I very quickly scored rankings on #1 with some long tail variations of it (not gonna reveal the niche but asume "big tits" and i scored for "big black tits"). I did not built any backlinks or whatnot. Just reblogging other content and adding a few hashtags and a proper description and title and maybe 100 characters text
The tumblr is now more or less on hold and i haven'T been posting anything for 6 months (never saw a good way to monetize it except for pennies, and i value my time

). Either way, it is still seeing roughly 500 visitors a day.