I've included links to a few relevant Quora threads here:

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!

This is a nice comment from one of your links: Luis Felipe Lanz, Intrapreneur, software architect, tec...

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.

That sounds uncomfortably familiar to us veterans of the '99 crash.

until adwords, what was the Google business model?

Three or four strong companies emerging from the carnage of thousands and thousands of others doesn't sound like any more now fun than it was the first time.

Wasn't it «being part of Stanford» originally?

Larry and Sergey started Google, at which time it was no longer a stanford research project, many years before AdWords.

In fact, they were very resistant to the idea of have any advertisements and were adamant in maintaining clean search results.

I don't know why they won't use nice looking ads from sites like Carbon or The Deck, which IMO add value rather than take away.

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.

Wordpress has also enabled lots of developers to make tons of money through templates and plugins. For example, woothemes makes millions/year selling wordpress templates.

Serious question: do they?

Yep: http://mixergy.com/woothemes-adriaan-pienaar/

I don't think Tumblr are making money at the moment. I mean, they might sell themes but that's hardly going to come close to their outgoings.

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.

They are making money but probably not much. David once said they would probably make millions if they put a single AdSense banner in the Dashboard, but they refuse to use regular advertising and want to make a more creative way of making money.

(Source: http://techcrunch.com/2011/02/22/founder-stories-karp-tumblr...)

If they could fetch $1 cpm and sell their entire inventory they could pull $1M/day.

Correction: $1M every four days http://techcrunch.com/2011/05/17/tumblr-pageviews-a-day/

According to Techsmart Posterious a Funding:$5.14M! There has to be some brains behind it somewhere.

http://www.crunchbase.com/company/posterous

Tumbler 40.3M! http://www.crunchbase.com/company/tumblr

I believe I saw something about Posterous adding affiliate codes to links unless you already have one on there.

http://news.ycombinator.com/item?id=1309403 "Posterous silently changes all external links to affiliate links"

Can this generate enough revenue to run the service?

I don't think so, but I also know that judicious use of caching and Amazon S3 can keep your costs relatively low.

With how terrible the Tumblr service has been the past year (I finally had to move every site I had off of it) I often wonder if they are making money. I had read somewhere they make pretty good on the themes and "feature" piece, but not sure how that could amount to very much.

Must be just you, Tumblr been running great for my 30+ blogs.

Point Pingdom at it and see if you still think that. I get at least one downtime notice a week.

I wonder how file hosting sites make money. Especially those without ads.

/threadjacker

Those with ads (like 4shared and megaupload) get money from annoyingly trying to get you to subscribe to get pirated content.

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.

Posterous: Add affiliate links, special marketing deals with brands

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.

Two of Tumblr's three sources of revenue (featured directory listings and digital stickers to put on directory listings) are no longer after they ditched the old directory system a few months ago in favor of a curated one.

Personally, I think if they just offered premium themes at a fair price they would make a killing.

they borrow it

For Posterous, custom domains are $13 (or maybe $25, the custom domain signup page is a little confusing) if you register through them. There's a fair amount of markup there.