there's an economics of information density and this is why twitter is gibberish and
blogs with google ads makes sense
basically, its some sort of entropic/SNR thing which is to do with the percentage of cognition someone is prepared to spend on stuff that isn't 100% germane (fancy word for relevant)
i think there's two pieces
1. there's a minimum unit's worth which is about 1 web page (8kbytes) - below this (think twitter or SMS), there's no useful semantic content, and its pure deontic/emoticon stuff - above this, the brain can afford some idling time to eyeball some stuff around the edges
so if you can't make money purely from the service (think sms/twitter) for low payload things you are doomed, but once you get into the page (think pagerank) you can build parasitic ways of making money (like advertising)...so in those cases, you want the _service_ to be free to maximise your chances of reaching the right people with the advert....(the advert could itself just be links to more content of course)
incidentally, this is why the IP MTU should have been 8Kbytes, not 576 (or the de facto 1500) it is today...