One thing, that you are not able to do, with this set up (Tumblr) is to have a mobile version of your site, which would be generating about 50% of the income. If you want to stay in this business, you need to work with programs like
Bitter Strawberry and
Sex Goes Mobile, and add something like mobile popunders from
Plugrush to the mix.
Other ad networks to use on mobile versions of your sites would be
First Mobile Cash, and
VPS Mobile, but I would recommend these for high traffic websites, are they are somehow more advanced.