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!
In my humble opinion, Posterous seems to be following the same strategy than Foursquare, Twitter or even Quora, start for free until get a consolided installed base users, then start side business around publicity.
In fact, they were very resistant to the idea of have any advertisements and were adamant in maintaining clean search results.
I just read that Tumblr has passed WordPress in terms of # of installed blogs, but I believe Matt's company makes tons more cash. Very interesting too, considering WP is OS.
I guess they're working on the revenue generating bit, as seems to be the case for so many funded tech companies over the last few years.
(Source: http://techcrunch.com/2011/02/22/founder-stories-karp-tumblr...)
Correction: $1M every four days http://techcrunch.com/2011/05/17/tumblr-pageviews-a-day/
/threadjacker
Yes, megavideo, I am looking at your 72-minutes limit.
About those without ads, probably their business model is to trying to get you to subscribe, by being less annoying than those without ads.
Tumblr: Premium themes, featured directory listings, "digital stickers"
They could probably earn a lot more if they started with ads or premium accounts. Right now it looks like they're focused on acquiring users, the focus on profits will likely come later.