http://www.quora.com/What-is-Posterous-business-model
http://www.quora.com/Posterous/How-does-Posterous-make-money
http://www.quora.com/Tumblr/How-does-Tumblr-make-money
http://www.quora.com/How-exactly-will-Tumblr-monetize-Other-...
http://www.quora.com/Tumblr/How-does-Tumblr-generate-revenue
http://www.quora.com/How-do-free-blog-publishing-platforms-e...
http://www.quora.com/How-do-you-monetize-blogging-as-the-blo...
There is surprisingly little detail here, though, possibly suggesting that the companies haven't quite figured it out yet!