Whenever I post something, I get around 1000 click through's, or "notes" as they call them in tumblr.
The user base is anywhere from 13 years old to 25 years old. Tumblr is very visual, they love appealing images (That's what makes them click through).
What I'm offering is to advertise for you, your product, image, brand, whatever it may be. (As long as it's not NSFW)
At only $5, come test this out, you will not be dissapointed.
The good thing with tumblr is that once something is posted, and people keep reblogging / liking the image, it's forever a viral object and will continue to populate more and more tumblr blogs.