If you administer or run a promotion (e.g., contest, sweepstakes, giveaway, etc.) on Tumblr, you must:
We reserve the right (but are not required) to investigate, suspend, and/or terminate any promotion we believe or suspect, in our sole discretion, may be fraudulent, unethical, or in violation of our policies, or any applicable laws. You assume all risk and liability for running any promotion.
Communicating on Tumblr about a promotion that happens off-site is generally OK (but make sure to provide all the information you need to, as noted above).
You will find a link to prior versions of our Promotions Guidelines below. We have stored these versions on GitHub, which will also allow you to compare different versions and see what terms have changed:
http://github.com/tumblr/policy/commits/master/contest-guidelines.txt