"Celebrities and big media sites" listed as key to tumblr's recent explosion? No, the secret is the service allowing a HUGE forest of porn compilation sites such as http://art-or-porn.tumblr.com/ and many like it.
Also, since I often see this mentioned as a difference, does Posterous do something to block porn (however strictly)?
I suppose my real question is "Sure, Tumblr has a lock on the porn market right now, but why?"
I guess you could argue erotica isn't porn, but yes, they like it. The notion actually extends beyond sexually explicit material, as many popular tumblelogs in the directory has porn or something that alludes to porn in their title even though they have nothing to do with sex.
I'm a big fan of Posterous but the social edge on Tumblr means more traffic and exposure for the blog which what attracts and keeps people on the platform.