my personal opinion is you don't ever teach critical mass to do anything. they will quickly ruin yet another avenue to gain traffic. you let the smart ones figure it out on their own, and i can tell you from using tumblr, it's not rocket science, and the ones with no restraint will never even pursue it. teaching the less than intelligent to do anything isn't really the best idea in my honest opinion. people who are really good at tumblr are not gonna be happy with stuff like this. 3,2,1 till tumblr promo is destroyed by halfwits.
just my personal opinion of course, and i'm sure some halfwit will be along any time to try to tell me i'm not entitled to it.
:2cents